Update SQL Conformance Documentation

In a discussion on dev list
http://apache-ignite-developers.2346864.n4.nabble.com/SparkDataFrame-Query-Optimization-Prototype-tt26249.html

was noticed there are some gaps in SQL documentation.
It's necessary to review it.

I've added next system functions:
CASEWHEN Function, CAST, CONVERT, TABLE

And for my mind, next functions aren't applicable for Ignite:
ARRAY_GET, ARRAY_LENGTH, ARRAY_CONTAINS, CSVREAD, CSVWRITE, DATABASE, DATABASE_PATH, DISK_SPACE_USED, FILE_READ, FILE_WRITE, LINK_SCHEMA, MEMORY_FREE, MEMORY_USED, LOCK_MODE, LOCK_TIMEOUT, READONLY, CURRVAL, AUTOCOMMIT, CANCEL_SESSION, IDENTITY, NEXTVAL, ROWNUM, SCHEMA, SCOPE_IDENTITY, SESSION_ID, SET, TRANSACTION_ID, TRUNCATE_VALUE, USER, H2VERSION