Add/finish API docs for Ontology methods

[jclerman]

Many of the methods in the Ontology class (ontol.py) have terse documentation that doesn't include a description of the argument-types. For example, add_synonym takes a single argument syn that is meant to be a Synonym object - not a simple string - but that's not obvious to a newcomer to the library, and requires examination of the source code.

Complete documentation would make the library much easier to learn/use.