Documentation: expand 'git diff' SEE ALSO section

Documentation: expand 'git diff' SEE ALSO section

Point in many directions in the hope of helping the reader find what
is needed more quickly.

This commit also removes the summary attached to the SEE ALSO entry
for difftool, to avoid making the SEE ALSO list too verbose.  If the
reader wants a summary of the commands referred to, she can always
look to the top of the named pages or to the table of contents on the
main git(1) page.

Suggested-by: Goswin von Brederlow <goswin-v-b@web.de>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>