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>