Improve comments for the default backtrace printer

The existing comments were misleading, confusing, and wrong.

Take this comment for example:
```
// Any frames between `__rust_begin_short_backtrace` and `__rust_end_short_backtrace`
// are omitted from the backtrace in short mode, `__rust_end_short_backtrace` will be
// called before the panic hook, so we won't ignore any frames if there is no
// invoke of `__rust_begin_short_backtrace`.
```

this is just wrong. here is an example (full) backtrace:

```
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.01s
     Running `/home/jyn/.local/lib/cargo/target/debug/example`
called `Option::unwrap()` on a `None` value
stack backtrace:
   0:     0x56499698c595 - std::backtrace_rs::backtrace::libunwind::trace::h5ef2cc16e9a7415a
   1:     0x56499698c595 - std::backtrace_rs::backtrace::trace_unsynchronized::h9b5e016e9075f714
   2:     0x56499698c595 - std::sys_common::backtrace::_print_fmt::h2f62c7f9ff224e93
   3:     0x56499698c595 - <std::sys_common::backtrace::_print::DisplayBacktrace as core::fmt::Display>::fmt::hbe51682735731910
   4:     0x5649969aa26b - core::fmt::rt::Argument::fmt::h1994ab2b310d665e
   5:     0x5649969aa26b - core::fmt::write::hade58a36d63468d7
   6:     0x56499698a43f - std::io::Write::write_fmt::h16145587d801a9ab
   7:     0x56499698c36e - std::sys_common::backtrace::_print::ha8082e56201dadb4
   8:     0x56499698c36e - std::sys_common::backtrace::print::he30f96b4e7f6cbfd
   9:     0x56499698d709 - std::panicking::default_hook::{{closure}}::hf0801f6b18a968d3
  10:     0x56499698d4ac - std::panicking::default_hook::hd2defec7eda5aeb0
  11:     0x56499698dc31 - std::panicking::rust_panic_with_hook::hde93283600065c53
  12:     0x56499698daf3 - std::panicking::begin_panic_handler::{{closure}}::h5e151adbdb7ec0c1
  13:     0x56499698ca59 - std::sys_common::backtrace::__rust_end_short_backtrace::he36a1407e0f77700
  14:     0x56499698d7d4 - rust_begin_unwind
  15:     0x5649969a9503 - core::panicking::panic_fmt::h2380d41365f95412
  16:     0x5649969a958c - core::panicking::panic::h38cf8db80e8c6e67
  17:     0x5649969a93e9 - core::option::unwrap_failed::he72696e53ff29a05
  18:     0x5649969722b6 - core::option::Option<T>::unwrap::hb574dc0dc1703062
  19:     0x5649969722b6 - example::main::h7a867aafacd93d75
  20:     0x5649969721db - core::ops::function::FnOnce::call_once::h734f99a5e57291b7
  21:     0x56499697226e - std::sys_common::backtrace::__rust_begin_short_backtrace::h02f5d58c351c4756
  22:     0x564996972241 - std::rt::lang_start::{{closure}}::h8b134fe2c31a4355
  23:     0x564996988662 - core::ops::function::impls::<impl core::ops::function::FnOnce<A> for &F>::call_once::h88d7bb571ee2aaf4
  24:     0x564996988662 - std::panicking::try::do_call::hfb78dfb6599c871d
  25:     0x564996988662 - std::panicking::try::habd041c8c4c8e50c
  27:     0x564996988662 - std::rt::lang_start_internal::{{closure}}::h227591a6f9c0879e
  28:     0x564996988662 - std::panicking::try::do_call::h3c5878333c38916a
  29:     0x564996988662 - std::panicking::try::h5af7b3a127cdae70
  31:     0x564996988662 - std::rt::lang_start_internal::hbc85e809eeace0dd
  32:     0x56499697221a - std::rt::lang_start::ha1eb16922c9cb224
  33:     0x5649969722ee - main
  34:     0x7f031962a1ca - __libc_start_call_main
  35:     0x7f031962a28b - __libc_start_main_impl
  36:     0x5649969720a5 - _start
  37:                0x0 - <unknown>
```

note particularly frames 13-21, from start_backtrace to end_backtrace. with PrintFmt::Short, these are the *only* frames that are printed; i.e. we are doing the exact opposite of the comment.

std/src/sys/backtrace.rs

@@ -58,8 +58,8 @@ unsafe fn _print_fmt(fmt: &mut fmt::Formatter<'_>, print_fmt: PrintFmt) -> fmt::
     let mut res = Ok(());
     let mut omitted_count: usize = 0;
     let mut first_omit = true;
-    // Start immediately if we're not using a short backtrace.
-    let mut start = print_fmt != PrintFmt::Short;
+    // If we're using a short backtrace, ignore all frames until we're told to start printing.
+    let mut print = print_fmt != PrintFmt::Short;
     set_image_base();
     // SAFETY: we roll our own locking in this town
     unsafe {
@@ -72,27 +72,25 @@ unsafe fn _print_fmt(fmt: &mut fmt::Formatter<'_>, print_fmt: PrintFmt) -> fmt::
             backtrace_rs::resolve_frame_unsynchronized(frame, |symbol| {
                 hit = true;
 
-                // Any frames between `__rust_begin_short_backtrace` and `__rust_end_short_backtrace`
-                // are omitted from the backtrace in short mode, `__rust_end_short_backtrace` will be
-                // called before the panic hook, so we won't ignore any frames if there is no
-                // invoke of `__rust_begin_short_backtrace`.
+                // `__rust_end_short_backtrace` means we are done hiding symbols
+                // for now. Print until we see `__rust_begin_short_backtrace`.
                 if print_fmt == PrintFmt::Short {
                     if let Some(sym) = symbol.name().and_then(|s| s.as_str()) {
-                        if start && sym.contains("__rust_begin_short_backtrace") {
-                            start = false;
+                        if sym.contains("__rust_end_short_backtrace") {
+                            print = true;
                             return;
                         }
-                        if sym.contains("__rust_end_short_backtrace") {
-                            start = true;
+                        if print && sym.contains("__rust_begin_short_backtrace") {
+                            print = false;
                             return;
                         }
-                        if !start {
+                        if !print {
                             omitted_count += 1;
                         }
                     }
                 }
 
-                if start {
+                if print {
                     if omitted_count > 0 {
                         debug_assert!(print_fmt == PrintFmt::Short);
                         // only print the message between the middle of frames
@@ -112,7 +110,7 @@ unsafe fn _print_fmt(fmt: &mut fmt::Formatter<'_>, print_fmt: PrintFmt) -> fmt::
             });
             #[cfg(target_os = "nto")]
             if libc::__my_thread_exit as *mut libc::c_void == frame.ip() {
-                if !hit && start {
+                if !hit && print {
                     use crate::backtrace_rs::SymbolName;
                     res = bt_fmt.frame().print_raw(
                         frame.ip(),
@@ -123,7 +121,7 @@ unsafe fn _print_fmt(fmt: &mut fmt::Formatter<'_>, print_fmt: PrintFmt) -> fmt::
                 }
                 return false;
             }
-            if !hit && start {
+            if !hit && print {
                 res = bt_fmt.frame().print_raw(frame.ip(), None, None, None);
             }