bug#52018: 28.0.60; Improve documentation for compilation-finish-functions

[Eli Zaretskii]

> From: Thibault Polge <thibault@thb.lt>
> Cc: 52018@debbugs.gnu.org
> Date: Sun, 21 Nov 2021 21:20:28 +0100
> 
> > The function is called with the compilation buffer, right?  And the
> > compilation process runs in that buffer, right?  So the function can
> > call get-buffer-process, and then access the exit status via
> > process-exit-status.
> 
> I actually tried that, but the docstring for get-buffer-process says the
> process must be live.  The following attempt fails:

OK, then the info is also on the mode line and can be taken from
there.