api: Redocument all of lily.h.
api: Redocument all of lily.h.

The new comments are meant to be processed by NaturalDocs to generate
api documentation. It is hoped that generating documentation this way
will prevent documentation from becoming stale.

FascinatedBox/lily,jesserayadkins/lily,jesserayadkins/lily,jesserayadkins/lily,FascinatedBox/lily,FascinatedBox/lily,FascinatedBox/lily