Update comments plugin documentation
Update comments plugin documentation

This factors in Akai's changes, though doesn't use the restructuring
he did.  It does move important information to the top of the documentation
and adds a note about how comments can't be used with static rendering.
It also reworks the Install instructions and makes note about configuration
and adding the flavour templates.

willkg/douglas,daitangio/pyblosxom,daitangio/pyblosxom,willkg/douglas