revisions.txt: use description list for special refs

revisions.txt: use description list for special refs

The special refs listed in 'gitrevisions' (under the '<refname>' entry)
are on separate lines in the Asciidoc source, but end up as a single
continuous paragraph in the rendered documentation (see e.g. [1]). In
following commits we will mention additional special refs, so to improve
legibility, use a description list such that every entry appears on its
own line. Since we are already in a description list, use ':::' as the
term delimiter.

In order for the new description list to be aligned with the description
under the '<refname>' entry, instead of being aligned with the last
entry of the "in the following rules" nested list, use the "ancestor
list continuation" syntax [2], i.e., leave an empty line before the
continuation '+'. Do the same for the paragraph following the new
description list ("Note that any...").

While at it, also use a continuation '+' before the "in the following
rules" list, for correctness. The parser seems not to care here, but
it's best to keep the sources correct.

[1] https://git-scm.com/docs/gitrevisions#Documentation/gitrevisions.txt-emltrefnamegtemegemmasterememheadsmasterememrefsheadsmasterem
[2] https://docs.asciidoctor.org/asciidoc/latest/lists/continuation/#ancestor-list-continuation

Suggested-by: Victoria Dye <vdye@github.com>
Based-on-patch-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Philippe Blain <levraiphilippeblain@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>