tersestatus: rework dirnode and tersedir docstrings Follow-up on refactorings 3d6d4b12128e and 5d98674df18a of the original changeset 7e3001b74ab3 by updating the docstrings of dirnode class and tersedir function: * rewrite dirnode.iterfilepaths()'s docstring (the method got renamed and reimplemented in 5d98674df18a); * simplify and update dirnode.tersewalk() to remove reference to 'self' and 'tersedict'; * use the imperative form of verbs in the first sentence of all docstrings. Differential Revision: https://phab.mercurial-scm.org/D1072