help: apply the section headings from revsets to filesets This has the nice property of visually breaking up the wall of text. It also allows specific smaller sections to be called out. For example, `hg help filesets.predicates` now prints just the predicate section. At the moment, the revset headings are a superset of the fileset headings, so there is consistency in how example, predicate and operator help is called out. The reference to `hg help patterns` was moved to the overview section, so that it isn't stuck in the examples section.