bug#45864: 27.1; hack-local-variables-hook is not documented in Emacs Manual

[Howard Melman <hmelman@gmail.com>]

I've been frustrated with a few packages that set
outline-regexp as a file local variable in a way I don't
prefer.  Since this is set after my major-mode hook is run I
didn't think I had a way to override these.  The Emacs
manual only describes how to affect this with the variable
enable-local-variables which isn't granular enough to fix
this. 

Today I learned about the hack-local-variables-hook which
allows me to correct these.  I'm surprised that it and
before-hack-local-variables-hook are not documented in the
Emacs Manual but are only mentioned in the Elisp Reference
Manual.  Unless I'm misunderstanding something they're meant
for users and not just for elisp programmers so they should
be in the Emacs Manual.

In GNU Emacs 27.1 (build 1, x86_64-apple-darwin19.5.0, Carbon Version 162 AppKit 1894.5)
of 2020-11-14 built on Traviss-Mac.local
Repository revision: 443027ae4946e227b8eb7754751eea62ab7f3386
Repository branch: HEAD
Windowing system distributor 'Apple Inc.', version 10.15.7
System Description:  Mac OS X 10.15.7

-- 

Howard