git-reset.txt: make modes description more consistent

git-reset.txt: make modes description more consistent

Currently, the structure of the individual mode entries is different
which makes it difficult to grasp the differences between the modes.
Also, the same items are named differently (e.g. <commit>, "the named
commit", "the given commit", "the commit being switched to").

Structure and word all mode entries consistently.

Signed-off-by: Michael J Gruber <git@drmicha.warpmail.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>