Document window functions: CORE-2090, CORE-2133, CORE-2823, CORE-2830, CORE-2869, CORE-3619, CORE-3620, CORE-3621.

git-svn-id: svn+ssh://svn.code.sf.net/p/firebird/code/firebird/trunk@57983 65644016-39b1-43b1-bf79-96bc8fe82c15