Architecture enhancements

A few enhancements to the architecture doc.

Change-Id: Ie3516a96f41639abf12f42e975427a0ece617268
Drop sitepackages=False from tox.ini

This never made sense, the default is False anyway. But distros
specifically patch tox.ini with sitepackages=True to test their packages
instead of stuff fetched from PyPI. Specifically settings this to it's
default only makes ditros life worse.

Change-Id: I3cbdc416c5114b49bc0b6a4b125c75c53eec23dd
Merge "Architecture enhancements"
Merge "Drop sitepackages=False from tox.ini"