Clean up code

The attached patch contains quite a few comment and style changes:

1. Remove $Id$ as suggested by coder review.
2. Add missing documentation comment.
3. Add and correct a few code comments.
4. Document all callback parameters.

A few changes to logic (or, rather, structure):

1. Format $term->name instead of using format_username().
2. Change apachesolr_term_solr_reindex() to accept and use parameters.
3. Change apachesolr_term_solr_reindex() to reindex specific bundles.
4. Change apachesolr_term_solr_reindex() to rethrow exceptions for the apachesolr module to handle.