docs/conf.py: Set default_role = 'any'.

This causes `symbol` syntax to be equivalent to :any:`symbol`, which is
in turn the easiest way to cross-reference an arbitrary symbol in the
docs:

http://www.sphinx-doc.org/en/stable/markup/inline.html#role-any

:any: requires at least Sphinx 1.3 (for reference, Ubuntu 16.03 ships
with 1.3.6, the latest 1.6.3).

Any many of our docs, `symbol` is misused to specify arguments to
functions, etc. Refactoring that is in progress. (CODECONVENTIONS
already specify proper syntax for both arguments and xrefs, based
on CPython conventions).