bug#9117: 24.0.50; Document menu keyword `:selected'

[Drew Adams]

Please document the menu keyword :selected.  Presumably the place for
this would be: `(elisp) Extended Menu Items', along with :visible,
:enable, etc.
 
In particular, document its relation with keywords such as :visible and
:enable - e.g., if not visible then :selected is not evaluated (or
whatever is the truth).
 
For example, I see code such as this in a 3rd-party library, and I
wonder what happens if `ls-lisp-dirs-first' is undefined.  The :visible
sexp here is protected OK (by `ls-lisp-var-p'), but if :selected is
eval'd even when :visible is nil then this would presumably raise an
error.
 
["Ignore Case" dired-sort-menu-toggle-ignore-case :style toggle
    :selected ls-lisp-ignore-case :active t
    :help "Ignore case in alphanumeric sorting"
    ;; supported only by (Emacs 21) ls-lisp library and local dired:
    :visible (ls-lisp-var-p 'ls-lisp-ignore-case)]

["Dirs First" dired-sort-menu-toggle-dirs-first :style toggle
    :selected ls-lisp-dirs-first :active t
    :help "List subdirectories first [last if reversed]"
    ;; supported only by (Emacs 21) ls-lisp library and local dired:
    :visible (ls-lisp-var-p 'ls-lisp-dirs-first)]
 
Anyway, the point is that there seems to be no documentation at all for
:selected, even though it is used in lots of core Lisp code and in
3rd-party code.

In GNU Emacs 24.0.50.1 (i386-mingw-nt5.1.2600)
 of 2011-07-11 on 3249CTO
Windowing system distributor `Microsoft Corp.', version 5.1.2600
configured using `configure --with-gcc (4.5) --no-opt --cflags
-Ic:/build/include'