bug#34911: 26.1; doc about lock file names

[Drew Adams]

I started from the doc of `dired-omit-files', which says:

  The default is to omit `.', `..', auto-save files and lock files.
                                                        ^^^^^^^^^^

And the default value is this: "^\\.?#\\|^\\.$\\|^\\.\\.$"

I can only guess that a lock file has either the same syntax as (the
default syntax of) an auto-save file: `#...#', or that prefixed by `.':
`.#...#'.  In fact, it's a stretch to guess even that, as the ^\.?#
doesn't necessarily match even an auto-save file (no # appended).

I don't see documented anywhere what a lock file name looks like.
I looked through both the Emacs manual and the Elisp manual.  Did I miss
it somehow, or is it defined/described nowhere?

Please let users know what how lock files are named.  Thx.  If this is
already done somewhere, please refer to that location from node (elisp)
`File Locks'.

Please also update the doc of `dired-omit-files' to make clear that its
default value only approximately matches auto-save files (and lock
files?), and that even this is true only for the default naming regime
for auto-save files.

In GNU Emacs 26.1 (build 1, x86_64-w64-mingw32)
 of 2018-05-30
Repository revision: 07f8f9bc5a51f5aa94eb099f3e15fbe0c20ea1ea
Windowing system distributor `Microsoft Corp.', version 10.0.17134
Configured using:
 `configure --without-dbus --host=x86_64-w64-mingw32
 --without-compress-install 'CFLAGS=-O2 -static -g3''