bug#26564: Document that only functions and not variables can end with "-p"

[Drew Adams]

> > I suggest we stay away from helper verbs (or even any words).
> > Verbs, in particular, can be different for singular and plural
> > (e.g. `fancy-candidates-p').
> 
> It's a matter of taste.  I like when symbols have names that speak to
> me, even when there is no clear rule of how to name them.

Yes.  But here we're talking about coming up with a general
naming convention (or not doing so).

> The most important thing is that you see how the binding is
> intended to be used.

Agreed.