revlog: rewrite several method docstrings
- methods: findmissing(), nodesbetween(), descendants(), ancestors()
- the goal is precise, concise, accurate, grammatical, understandable,
consistently formatted docstrings
.. Common link and substitution definitions.
.. |hg(1)| replace:: **hg**\ (1)
.. _hg(1): hg.1.html
.. |hgrc(5)| replace:: **hgrc**\ (5)
.. _hgrc(5): hgrc.5.html
.. |hgignore(5)| replace:: **hgignore**\ (5)
.. _hgignore(5): hgignore.5.html