doc/diff-options: fix out of place mentions of '--patch/-p'

doc/diff-options: fix out of place mentions of '--patch/-p'

First, references to --patch and -p appeared in the description of
git-format-patch, where the options themselves are not included.

Next, the description of --unified option elsewhere had duplicate implied
statements: "Implies --patch. Implies -p."

Signed-off-by: Sergey Organov <sorganov@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>