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

[Eli Zaretskii]

> From: Ram Bhamidipaty <rbhamidipaty@roku.com>
> CC: "22220@debbugs.gnu.org" <22220@debbugs.gnu.org>
> Date: Wed, 23 Dec 2015 17:54:12 +0000
> 
> There is also a slight problem with the set-process-sentinel docs - there 
> really isn't anything that says a process starts with a default sentinel - 
> even if the set-process-sentinel function is not called.

It's okay: the change I've just committed adds that as well.

> The current docs set-process-sentinel docs just say what happens if the 
> sentinel argument is nil. Careful reading certainly implies that the default 
> sentinel will be given to a process - but surely this kind of information can 
> be more explicit.

It is now.