The just released SQuirrel SQL 4.0.0 provides an Elasticsearch driver
definition out of the box; update the documentation to reflect that.
(cherry picked from commit 3aa417ed74947e69f0ff605b1c210a0678a3cb9f)
* Added Limitations page
* Made the aggregations page follow the common template for functions
* Modified all tables to have the first row's cells content centered
* Polishing in other various sections