Documentation: customize diff-options depending on particular command

Documentation: customize diff-options depending on particular command

Customize diff-options depending on particular command as follows,
mostly to make git-diff and git-format-patch manuals less confusing:

* git-format-patch:

  - Mark --patch-with-stat as being the default.

  - Change -p description so that it matches what it actually does and
    so that it doesn't refer to absent "section on generating
    patches".

* git-diff: mark -p as being the default.

* git-diff-index/git-diff-files/git-diff-tree: mark --raw as being
  the default.

Signed-off-by: Sergei Organov <osv@javad.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>