aborts: Clarify documentation and comments

[ijackson]

In the docs for intrinsics::abort():

• Strengthen the recommendation by to use process::abort instead.
• Document the fact that it sometimes (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

Fixes #40230