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

[Kaushal Modi <kaushal.modi@gmail.com>]

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The discussion on whether variables should end in "-p" came up few time on
emacs-devel, at least the ones I was part of:

1. https://lists.gnu.org/archive/html/emacs-devel/2016-07/msg00734.html
2. http://lists.gnu.org/archive/html/emacs-devel/2017-04/msg00540.html

Doing a rudimentary search in emacs/lisp/ code-base shows a sign of an
undocumented convention that predicate functions, functions that return
either nil or non-nil can end in "-p" (if the function name is multi-word,
like nested-alist-p), or just "p" (if the function name is a single word,
like stringp).

I ran ag[1] in emacs/lisp:

1. 1511 matches -- defuns and defsubsts ending in -p -- ag
'def(un|subst)\s+[^ ]+-p\s+' --stats
2. 149 matches -- defvars, defconsts and defcustoms ending in -p -- ag
'def(var|const|custom)\s+[^ ]+-p\s+' --stats
3. 65 matches -- Just the user-facing defcustoms -- ag 'defcustom\s+[^
]+-p\s+' --stats

That corroborates that more functions tend to end in "-p" than variables.

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?

[1]: https://github.com/ggreer/the_silver_searcher
-- 

Kaushal Modi

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