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 17:45:22 +0000
> 
> This is what I ran into: I am working on some code using the emacs process 
> facility. And I started seeing messages (from the default sentinel) that I 
> did not expect. When I _skimmed_ through the sentinel section I did not 
> notice that a default sentinel is added to all processes. I acknowledge that 
> skimming the docs was not the right thing to do - since I missed the fact 
> that there is a default sentinel.

OK, I already have a patch that will mention the default sentinel in a
few more places, and I think it will fit that bill.  I will commit it
soon.

Thanks.