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

[Eli Zaretskii]

> From: Kaushal Modi <kaushal.modi@gmail.com>
> Date: Wed, 19 Apr 2017 15:47:06 +0000
> 
> This bug report is to make a request to incorporate this unspoken rule as a 
> rule in the Elisp manual.
> 
> What would be the correct section to do so?

"Coding Conventions", of course.

Thanks.