bug#32016: 27.0; (elisp) `Imenu'

[Drew Adams]

The description of `imenu-generic-expression' is incomplete.  It does
not specify the case where argument REGEXP is a function.

The doc string describes this use case this way:

REGEXP may also be a function, called without arguments.  It is expected
to search backwards.  It must return true and set 'match-data' if it
finds another element.

Unfortunately, the last sentence there is unclear.  The function does
not "find another element".  It finds a definition construct in the
buffer, and it sets match data for it.  It does not construct or "find"
an element of the list that is the variable value.

But this bug report is about the missing description of the
REGEXP-as-function case.

In GNU Emacs 27.0.50 (build 3, x86_64-w64-mingw32)
 of 2018-03-21
Repository revision: e70d0c9e66d7a8609450b2889869d16aeb0363b5
Windowing system distributor `Microsoft Corp.', version 6.1.7601
Configured using:
 `configure --without-dbus --host=x86_64-w64-mingw32
 --without-compress-install -C 'CFLAGS=-O2 -static -g3''