docs: add missing docs for matchers.Matcher.format_any_of.

* docs/specl.md (Custom Matchers): Document format_any_of
constructor method.
* NEWS: Update.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>
docs: match <h2> anchor shortcuts with github.io generated html anchors.

* docs/specl.md: The javascript at http://gvvaughan/github.io/specl
rewrites h2 anchors with '-' separators.  Make sure the markdown
shortcuts match.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>