Use python docstrings

[Marco Giusti]

Python docstrings are special strings meant to be documentation for the
code. They are accessible by code (via `__doc__` attribute) or `pydoc`,
the python documentation tool.

While they are already many documentation strings, could be useful to
transform them in docstrins but not doing this will not have any
consequences. Quoting pep-257: If you violate these conventions, the
worst you'll get is some dirty looks.

 affects virtualbrick
 tag code-cleanup

[Marco Giusti (marco-giusti)]

Invalid: too generic.