Improved Citations in LdaModel

Added more detailed information about the papers referenced in the comments of LdaModel, including titles and author names.  

This change effectively:
- Clarifies the reference to the "Lee & Seung trick" in the implementation of `LdaModel.inference`
- Protects the reference to a paper about Dirichlet parameter estimation against a dead link in the future.  No author name or title was previously provided.

Updated references now appear in the docstrings of relevant functions.  It may be useful to add them to the module docstring as well.