restore flag docs to Goals Reference

builddict.py wants to know how a phase computes its OptionGroup's title. From this, it figures out that the Goals Reference entry for "compile" should show "jvm" options but not "ivy" options (available at the command line, but documented with "resolve" instead).

In the Phase refactor, this broke. Phase used "task" or "phase:task"; builddict used "phase.task", d'oh. If we generate the docs today, the Goals Reference is missing flags.

This change adds Phase.options_group_title() and uses it in Phase and in builddict.

Testing Done:
https://travis-ci.org/pantsbuild/pants/builds/32483970

Reviewed at https://rbcommons.com/s/twitter/r/885/