Documentation: document show -s

Documentation: document show -s

Git's diff machinery has supported a -s (silence diff output) option
as far back as v0.99~900 (Silent flag for show-diff, 2005-04-13), but
the option is only advertised in an odd corner of the git diff-tree
manual.

The main use is to retrieve basic metadata about a commit:

	git show -s rev

Explain this in the 'git log' manual and provide an example in the
'git show' examples section.  This is kind of a cop-out, since it
would be more useful to explain it in the 'git show' manual proper,
which says:

	The command takes options applicable to the git
	diff-tree command to control how the changes the
	commit introduces are shown.

	This manual page describes only the most frequently
	used options.

Fixing that is a larger task for another day.

Reported-by: Will Hall <will@gnatter.net>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>