Ref Guide: MoreLikeThis docs need overhaul

No offense to anyone who's worked on morelikethis.adoc, but the current docs are pretty vague and require a ton of reading between the lines to understand. I had to help someone with something today and it took me a couple hours of trying things out in a local instance just to wrap my head around it. Good docs would prevent that.

Some things off the top of my head that would help:

• examples of config and output for each approach (search component/request handler/query parser)
• documenting the defaults for each parameter
• moving the MoreLikeThisQParser docs from other-parsers.adoc to morelikethis.adoc for a single source for all MLT stuff in Solr