bug#63875: 26.3; Doc of `dired-guess-shell-alist-user'

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

The doc string doesn't say what REGEXP is, e.g., what it's intended to
match.  It should say that up front and explicitly.

Manual (dired-x) `Shell Command Guessing' sort of says it, if you read
carefully and guess a bit.

Please say explicitly that REGEXP is a regexp that's used to match file
names.  And say how the alist entries are used.  All that's said, so
far, is that this is a "User-defined alist of rules for suggested
commands."

Rules used how?  What does matching file names have to do with using a
command?  Again, the manual does say this somewhat, but not the doc
string.

The doc refers to `dired-guess-shell-alist-default', but the doc for
that variable doesn't explain anything, so it doesn't help with the
missing info for `dired-guess-shell-alist-user'.

FWIW, the comment in file dired-x.el that introduces these variables
describes them better than the doc in some ways: what they're for, how
they're used.

In GNU Emacs 26.3 (build 1, x86_64-w64-mingw32)
 of 2019-08-29
Repository revision: 96dd0196c28bc36779584e47fffcca433c9309cd
Windowing system distributor `Microsoft Corp.', version 10.0.19045
Configured using:
 `configure --without-dbus --host=x86_64-w64-mingw32
 --without-compress-install 'CFLAGS=-O2 -static -g3''