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.