Documentation: move description of -s, --no-patch to diff-options.txt

Technically, "-s, --no-patch" is implemented in diff.c ("git diff
--no-patch" is essentially useless, but valid). From the user point of
view, this allows the documentation to show up in "git show --help",
which is one of the most useful use of the option.

While we're there, add a sentence explaining why the option can be
useful.

Signed-off-by: Matthieu Moy <Matthieu.Moy@imag.fr>
Reviewed-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/diff-options.txt

@@ -26,6 +26,11 @@ ifndef::git-format-patch[]
 	{git-diff? This is the default.}
 endif::git-format-patch[]
 
+-s::
+--no-patch::
+	Suppress diff output. Useful for commands like `git show` that
+	show the patch by default, or to cancel the effect of `--patch`.
+
 -U<n>::
 --unified=<n>::
 	Generate diffs with <n> lines of context instead of

Documentation/rev-list-options.txt

@@ -837,8 +837,4 @@ options may be given. See linkgit:git-diff-files[1] for more options.
 -t::
 
 	Show the tree objects in the diff output. This implies '-r'.
-
--s::
---no-patch::
-	Suppress diff output.
 endif::git-rev-list[]