bug#24353: 25.1.1: looking-back wrong info

[Drew Adams]

FWIW, I agree with Andreas.  Or at least I have a question as
to why this was changed (from longstanding practice for this
function's doc).

The *only* difference I see in the output of `C-h f
looking-back', between Emacs 24.5 and my most recent pre-Emacs
25 snapshot (which is not very recent, admittedly) is the false
signature (looking-back REGEXP LIMIT &optional GREEDY) instead
of the correct signature (looking-back REGEXP &optional LIMIT
GREEDY).

How does this change help Emacs users?

This is not a command, for users.  It is a function used by
Emacs-Lisp programmers.  Sounds like `C-h f' is becoming less
useful, and programmers now need to bypass it and go directly
to the source code, just to find out the correct signature.

If it is so important that people provide LIMIT then spell
that out _explicitly_ in the doc as a recommendation.  It
should be a no-no to just change the advertized signature of
a function, without changing the actual signature (code) and
without otherwise changing the doc.