Updates to MongoDB comparison table

The comparison table to MongoDB http://rethinkdb.com/docs/comparison-tables/ is outdated in many points

• MongoDB has added text indexes

• MongoDB now offers a different storage engine. The following statements are no longer true:

  OS-level memory mapped files LRU
  [...]
  MongoDB uses memory mapped files where the OS is in control of flushing writes and paging data in and out.

  ...and the section on "Caching engine"

• The supported platforms have changed. We added support for Windows, CentOS 7 (64 bit only) and discontinued Ubuntu 10.04 (the minimum supported version is now 12.04). I don't know about MongoDB.

• This is still true, but will change with RethinkDB 2.4:

  Neither MongoDB nor RethinkDB support incremental map-reduce by default.

• We now have official performance numbers: http://rethinkdb.com/docs/2-1-5-performance-report/

  Official performance numbers for RethinkDB have not been published yet. (We’re still working on improving performance for a number of specialized workloads.)

I'm not sure how important this page still is for interested users. It might still matter, but I think that it's probably less relevant than it used to be because our positioning is a lot more unique. @coffeemug, @mglukhovsky any thoughts on this?

Start with the comparison table at http://rethinkdb.com/docs/comparison-tables/ and review each outdated MongoDB, platform, map-reduce, caching, and performance statement against current facts. Done means the inaccurate claims are updated and the page's continued relevance is decided, with the documented comparison reflecting that decision.