Added notes about metadata to docs.
Added notes about metadata to docs.

* Added a note about metadata keys with no value.
* Added a note about how metadata is not stored in a multidict and
  that using the same key twice in the same entry will cause the
  second value to override the first one.

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