document clear_query_cache

Attached find a small but useful documentation path. Explains how and when to use the previously-undocumented clear_query_cache method, including discussion of an edge case where direct invocation of the method would be useful.

See this ticket: http://dev.rubyonrails.org/ticke... (The resolution of which was to determine that a doc patch is in order.)

[Sorry for second posting. My first ticket was marked as spam by lighthouse for some reason.]