Documentation: format-patch --root clarifications

Documentation: format-patch --root clarifications

Users were confused about the meaning and use of the --root option.
Notably, since 68c2ec7 (format-patch: show patch text for the root
commit, 2009-01-10), --root has nothing to do with showing the patch
text for the root commit any more.

Shorten and clarify the corresponding paragraph in the DESCRIPTION
section, document --root under OPTIONS, and add an explicit note that
root commits are formatted regardless.

Signed-off-by: Thomas Rast <trast@student.ethz.ch>
Signed-off-by: Junio C Hamano <gitster@pobox.com>