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

[Michael Heerdegen]

Drew Adams <drew.adams@oracle.com> writes:

> A variable whose value is a _predicate_ I'd name something like
> `foo-predicate' or `foo-pred' or even `foo-function' (the last one
> following the current convention).

For "exported" defvar'd variables I agree.