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