Rollup merge of #65633 - Rantanen:doc-example-paths, r=Centril

Remove leading :: from paths in doc examples

Noted some pre-2018 path syntax in the doc examples, for example:
https://doc.rust-lang.org/std/process/fn.exit.html

```rust
fn main() {
    ::std::process::exit(match run_app() {
       Ok(_) => 0,
       ...
```

Couldn't find an existing issue on this (then again, "::" makes for an annoying thing to search for) so if there is already something fixing this and/or there's a reason to not fix it, just close this PR.

(Also fixed indentation in the `process::exit()` docs)

src/libcore/iter/traits/collect.rs

@@ -167,7 +167,7 @@ pub trait FromIterator<A>: Sized {
 /// // and we'll implement IntoIterator
 /// impl IntoIterator for MyCollection {
 ///     type Item = i32;
-///     type IntoIter = ::std::vec::IntoIter<Self::Item>;
+///     type IntoIter = std::vec::IntoIter<Self::Item>;
 ///
 ///     fn into_iter(self) -> Self::IntoIter {
 ///         self.0.into_iter()

src/libcore/ops/unsize.rs

@@ -76,7 +76,7 @@ impl<T: ?Sized+Unsize<U>, U: ?Sized> CoerceUnsized<*const U> for *const T {}
 /// ```
 /// # #![feature(dispatch_from_dyn, unsize)]
 /// # use std::{ops::DispatchFromDyn, marker::Unsize};
-/// # struct Rc<T: ?Sized>(::std::rc::Rc<T>);
+/// # struct Rc<T: ?Sized>(std::rc::Rc<T>);
 /// impl<T: ?Sized, U: ?Sized> DispatchFromDyn<Rc<U>> for Rc<T>
 /// where
 ///     T: Unsize<U>,

src/libcore/str/mod.rs

@@ -176,15 +176,15 @@ Section: Creating a string
 /// ```
 /// fn from_utf8_lossy<F>(mut input: &[u8], mut push: F) where F: FnMut(&str) {
 ///     loop {
-///         match ::std::str::from_utf8(input) {
+///         match std::str::from_utf8(input) {
 ///             Ok(valid) => {
 ///                 push(valid);
 ///                 break
 ///             }
 ///             Err(error) => {
 ///                 let (valid, after_valid) = input.split_at(error.valid_up_to());
 ///                 unsafe {
-///                     push(::std::str::from_utf8_unchecked(valid))
+///                     push(std::str::from_utf8_unchecked(valid))
 ///                 }
 ///                 push("\u{FFFD}");
 ///

src/libstd/net/udp.rs

@@ -202,7 +202,7 @@ impl UdpSocket {
     ///
     /// let socket = UdpSocket::bind("127.0.0.1:34254").expect("couldn't bind to address");
     /// assert_eq!(socket.peer_addr().unwrap_err().kind(),
-    ///            ::std::io::ErrorKind::NotConnected);
+    ///            std::io::ErrorKind::NotConnected);
     /// ```
     #[stable(feature = "udp_peer_addr", since = "1.40.0")]
     pub fn peer_addr(&self) -> io::Result<SocketAddr> {

src/libstd/process.rs

@@ -1488,12 +1488,12 @@ impl Child {
 /// }
 ///
 /// fn main() {
-///     ::std::process::exit(match run_app() {
-///        Ok(_) => 0,
-///        Err(err) => {
-///            eprintln!("error: {:?}", err);
-///            1
-///        }
+///     std::process::exit(match run_app() {
+///         Ok(_) => 0,
+///         Err(err) => {
+///             eprintln!("error: {:?}", err);
+///             1
+///         }
 ///     });
 /// }
 /// ```