The new --reference flag introduced to git-clone in
GIT 1.3.0 was not documented but is rather handy.
So document it.

Also corrected a minor issue with the documentation for the
-s flag; the info/alternates file name was spelled wrong.
Signed-off-by: NShawn O. Pearce <spearce@spearce.org>
Signed-off-by: NJunio C Hamano <junkio@cox.net>