bug#29263: 26.0.90; Regression in doc-string highlighting

[Drew Adams <drew.adams@oracle.com>]

 (defun foo ()
  "Why does this show highlighting \* \( \) ??"
  nil)

Starting with Emacs 26 this highlights the backslashes differently from
the rest of the doc string (using face `font-lock-warning-face').

This is totally inappropriate.  Emacs is trying to be too clever here.
A backslash before `*' or a paren can mean any number of different
things in a doc string.

Consider this bit of a doc string, for instance:

 When both optional args are non-nil, the `find' command run is this:

   find . -mindepth MIN-DEPTH -maxdepth MAX-DEPTH
          \\( -path \*edir1\* -o -path \*edir2\* ... \\)
          -prune -o \\( ARGS \\) LS-SWITCHES"

It makes zero sense for `\*' here to have its backslash highlighted.
That just spreads confusion.  Any number of other examples can be
imagined.  A doc string is, above all, arbitrary natural-language text.
Trying to impose some clever semantics on backslashes is misguided.

Please revert this regressive behavior.


In GNU Emacs 26.0.90 (build 3, x86_64-w64-mingw32)
 of 2017-10-13 built on LAPHROAIG
Repository revision: 906224eba147bdfc0514090064e8e8f53160f1d4
Windowing system distributor 'Microsoft Corp.', version 6.1.7601
Recent messages:
For information about GNU Emacs and the GNU system, type C-h C-a.
Saving file d:/usr/drew/drews-lisp-20/1111.el...
Wrote d:/usr/drew/drews-lisp-20/1111.el
Type C-x 1 to delete the help window, C-M-v to scroll help.
Char: \ (92, #o134, #x5c) point=142 of 41521 (0%) column=35

Configured using:
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 --without-compress-install 'CFLAGS=-O2 -static -g3''

Configured features:
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