bug#33443: 26; Doc of Edit Abbrevs mode

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

In `edit-abbrevs-mode' I hit `C-h m' to find out about the mode, what
I'm seeing in the buffer, and what I can do with it.

This is all I get, which I feel is not very useful:

 Edit-Abbrevs mode defined in `abbrev.el' (`edit-abbrevs-mode'):
 Major mode for editing the list of abbrev definitions.

 This mode runs the hook `edit-abbrevs-mode-hook', as the final or penultimate step
 during initialization.

 key             binding
 ---             -------

 C-c             Prefix Command
 C-x             Prefix Command

 C-c C-c         edit-abbrevs-redefine

 C-x C-s         abbrev-edit-save-buffer
 C-x C-w         abbrev-edit-save-to-file

[One of those help lines is 82 chars long, BTW.]

You have to roam around in the source file a bit to find out what you're
looking at and what you can do with it.  In particular, the doc of
`edit-abbrevs' tells you what you're looking at and what you can do with
what you're looking at. It is...

 a buffer containing a list of abbrev definitions with
 point located in the abbrev table of current buffer.
 You can edit them and type C-c C-c to redefine abbrevs
 according to your editing.
 Buffer contains a header line for each abbrev table,
  which is the abbrev table name in parentheses.
 This is followed by one line per abbrev in that table:
 NAME   USECOUNT   EXPANSION   HOOK
 where NAME and EXPANSION are strings with quotes,
 USECOUNT is an integer, and HOOK is any valid function
 or may be omitted (it is usually omitted).

This description is pretty summary, lacking explanation
about abbrevs, but it is certainly better than what
`edit-abbrevs-mode' tells you.

("Current buffer" is wrong here, BTW, as what is meant
by it is not the buffer of buffer *Abbrevs* but the
buffer that you came from to create buffer *Abbrevs*.)

You can get to an Edit-Abbrev mode buffer in several ways.
Once you are there `C-h m' should tell you what it is and what you can
do with it.

Similarly, the doc in the Emacs manual (node `Abbrev Concepts' is not
very helpful.  It tells you how you can use (expand) an existing abbrev,
but it says nothing about how to create an abbrev, for instance.


In GNU Emacs 26.1 (build 1, x86_64-w64-mingw32)
 of 2018-05-30
Repository revision: 07f8f9bc5a51f5aa94eb099f3e15fbe0c20ea1ea
Windowing system distributor `Microsoft Corp.', version 10.0.16299
Configured using:
 `configure --without-dbus --host=x86_64-w64-mingw32
 --without-compress-install 'CFLAGS=-O2 -static -g3''