doc: flatten description of 'matching()' predicate to be formatted well
current description of 'matching()' revset predicate can't be
formatted well on "hg help revset" output.
each descriptions for revset predicates (or something like them) are
split-ed into lines, and spaces on left side of them are stripped
before minirst processing. so, bullet list can't be nested.
this patch just flattens description of 'matching()' predicate to be
formatted well.