Enumerate revision range specifiers in the documentation

Enumerate revision range specifiers in the documentation

It was a bit hard to learn how <rev>^@, <rev>^! and various other
forms of range specifiers are used, because they were discussed
mostly in the prose part of the documentation, unlike various forms
of extended SHA-1 expressions that are listed in an enumerated list.

Also add a few more examples showing use of <rev>, <rev>..<rev> and
<rev>^! forms, stolen from a patch by Max Horn.

Signed-off-by: Junio C Hamano <gitster@pobox.com>