gendoc: use an empty comment so aliases are separated from previous elements
For commands like hg bookmarks, where there's no `[+] marked option can be
specified multiple times`, this causes the final option in the option list to
not be the parent of the aliases definition. The aliases section is thus marked
as a blockquote like on commands that do have text separating the option list
and the aliases definition.
Differential Revision: https://phab.mercurial-scm.org/D9990