bug#31698: 27.0; `rx' help: Show equivalent regexp constructs

[Drew Adams]

Help for `rx' could use some improvement.

1. There seems to be no other help for `rx' than `C-h f rx'.  Nothing in
   the Elisp manual, for instance.  Perhaps it should have its own
   manual.  Or perhaps it should be documented in the Elisp manual (?).
   It's hard to imagine someone trying to learn the use of `rx' just by
   looking at `C-h f rx'.  Emacs should try to do better.

2. Please document (in the doc string of `rx', if nowhere else) the
   correspondences between each of the `rx' constructs and regexp
   syntax.  At least please document the most important ones.  For
   example, `zero-or-more' presumably corresponds to postfix regexp char
   `*'.

3. Please consider reordering the doc-string text to cover more commonly
   used and more important constructs before those less likely to be
   used.  E.g., `not', `and', and `or', seem more common and more
   important than `category'.

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''