doc: revisions: give headings for the two and three dot notations
While there, also break out the other shorthand notations and
add a title for the revision range summary (which also appears
in git-rev-parse, so keep it mixed case).
We do not quote the notation within the headings as the asciidoc ->
docbook -> groff man viewer toolchain, particularly the docbook-groff
step, does not cope with two font changes, failing to return the heading
font to bold after the quotation of the notation.
Signed-off-by: Philip Oakley <philipoakley@iee.org> Signed-off-by: Junio C Hamano <gitster@pobox.com>