stash: mark "git stash save" deprecated in the man page

stash: mark "git stash save" deprecated in the man page

'git stash push' fixes a historical wart in the interface of 'git stash
save'.  As 'git stash push' has all functionality of 'git stash save',
with a nicer, more consistent user interface deprecate 'git stash
save'.  To do this, remove it from the synopsis of the man page, and
move it to a separate section, stating that it is deprecated.

Helped-by: Robert P. J. Day <rpjday@crashcourse.ca>
Helped-by: Jeff King <peff@peff.net>
Signed-off-by: Thomas Gummerer <t.gummerer@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>