Improve javadocs for public SolrJ classes

While poking around some SolrJ code, I noticed that the Javadoc documentation tends to be spotty. Some sections have pretty meticulous descriptions, others are missing javadocs entirely.

I'm not aiming to entirely correct that situation here, but I did want to fix a few of the more serious concerns I ran into in some of my digging. This list includes:

• SolrClient.commit should have some warning about the downside of invoking commits on the client side
• ditto re: SolrClient.rollback
• SolrClient's single-doc add method should have a warning about performance implications of not batching. Not sure if this should live in SolrClient itself and be worded as a "potential" perf impact, or live in each of the clients it applies to.
• the SolrClient builders can use some clarification around when particular settings are useful.
• ResponseParser and some other classes might benefit from some high level class javadocs.

Figured this was worth a JIRA so others can catch potential mistakes I'm making here, or suggest other SolrJ things that'd really benefit from Javadocs.