aborts: Clarify documentation and comments

In the docs for intrinsics::abort():

 * Strengthen the recommendation by to use process::abort instead.
 * Document the fact that it (ab)uses an LLVM debug trap and what the
   likely consequences are.
 * State that the precise behaviour is unstable.

In the docs for process::abort():

 * Promise that we have the same behaviour as C `abort()`.
 * Document the likely consequences, including, specifically, the
   consequences on Unix.

In the internal comment for unix::abort_internal:

 * Refer to the public docs for the public API functions.
 * Correct and expand the description of libc::abort.  Specifically:
 * Do not claim that abort() unregisters signal handlers.  It doesn't;
   it honours the SIGABRT handler.
 * Discuss, extensively, the issue with abort() flushing stdio buffers.
 * Describe the glibc behaviour in some detail.

Co-authored-by: Mark Wooding <mdw@distorted.org.uk>
Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>

library/std/src/process.rs

@@ -1908,6 +1908,10 @@ pub fn exit(code: i32) -> ! {
 /// this function at a known point where there are no more destructors left
 /// to run.
 ///
+/// The process's termination will be similar to that from the C `abort()`
+/// function.  On Unix, the process will die with signal `SIGABRT`, which
+/// typically means that the shell prints "Aborted".
+///
 /// # Examples
 ///
 /// ```no_run