bug#24897: 24.5; doc for `M' in Dired

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

This: http://emacs.stackexchange.com/q/28504/105, asks how to make files
read-only using Dired.  Besides using wdired, the obvious answer is to
use `M'.

However, in answering that, it becomes clear that the questioner does
not know the UNIX command `chmod', and might not know about file and
directory permissions.  S?he might not even have what it takes locally
to use `M-x man', to find out.  And the doc for `M' - both the doc
string and the Emacs manual, does not really explain `chmod' or what
kind of input a user can type to the prompt from `M'.

I think it would be helpful, for at least some users, to add some more
explanation.  In particular, it could explain about u, g, o, and a, and
about the permissions fields rwx and their values.


In GNU Emacs 24.5.1 (i686-pc-mingw32)
 of 2015-04-11 on LEG570
Windowing system distributor `Microsoft Corp.', version 6.1.7601
Configured using:
 `configure --prefix=/c/usr --host=i686-pc-mingw32'