Add index feature support matrix documentation for different backends

Currently the mixed index documentation is heavily ES centric.

Some examples:
a) http://docs.janusgraph.org/0.1.0/index-parameters.html
21.1.3. Full text and string search
**"If you are using ElasticSearch it is possible to..."**
b) http://docs.janusgraph.org/0.1.0/direct-index-query.html
22.1. Query String
**"Elasticsearch and Lucene support ..."**
c) http://docs.janusgraph.org/0.1.0/search-predicates.html
20.7. Collections
**"If you are using Elasticsearch then you can index properties with SET and LIST cardinality"**

This does not mention Solr anywhere leaving anyone trying to use those features with Solr wondering if they actually work.
Even if they do NOT work if would be better to get this information explicitly from the docs, and not by trial and error.

This could be improved by e.g. providing a feature matrix support table with different backends stated in the columns and features states in the rows.

Start with the index-parameters, direct-index-query, and search-predicates documentation sections linked in the issue. Check the documented feature behavior for each supported index backend, including Solr, against the relevant backend capabilities. Done means the docs include a clear support matrix and explicitly identify unsupported or unverified combinations.

Written by the indexing model from the issue text.