Document that CStr::as_ptr returns a type alias

Workaround for rust-lang#15823

library/core/src/ffi/c_str.rs

@@ -457,6 +457,10 @@ impl CStr {
  /// to a contiguous region of memory terminated with a 0 byte to represent
  /// the end of the string.
  ///
+ /// The type of the returned pointer is
+ /// [`*const c_char`][crate::ffi::c_char], and whether it's
+ /// an alias for `*const i8` or `*const u8` is platform-specific.
+ ///
  /// **WARNING**
  ///
  /// The returned pointer is read-only; writing to it (including passing it
@@ -470,6 +474,7 @@ impl CStr {
  /// # #![allow(unused_must_use)] #![allow(temporary_cstring_as_ptr)]
  /// use std::ffi::CString;
  ///
+ /// // Do not do this:
  /// let ptr = CString::new("Hello").expect("CString::new failed").as_ptr();
  /// unsafe {
  /// // `ptr` is dangling