pep257: write docstrings as a command

documented for one-line docstrings:
http://legacy.python.org/dev/peps/pep-0257/#one-line-docstrings

this commit *only* concerns 3rd-person forms
to reduce changes to a minimum:
even obvious typos within the same docstrings have *not* been fixed

Change-Id: I8c6e51e02904d499f30f51fe063d1b2e17ea7f67