bug#22220: 24.5; no documentation for default process sentinel

[Eli Zaretskii]

> From: Ram Bhamidipaty <rbhamidipaty@roku.com>
> Date: Tue, 22 Dec 2015 01:20:28 +0000
> 
> Emacs has the concept of a default process sentinel - but this is not
> documented in the emacs lisp reference manual.

The default sentinel is documented with the set-process-sentinel
function:

 -- Function: set-process-sentinel process sentinel
     This function associates SENTINEL with PROCESS.  If SENTINEL is
     `nil', then the process will have the default sentinel, which
     inserts a message in the process's buffer when the process status
     changes.

Maybe this is not prominent enough, and we should say this in a few
more places.  Other than that, what else do you think should be said
about the default sentinel?

Thanks.