Adds doc string to query validate functions in V2 API

Rename is_timestamp_valid parameter to allow_timestamps and add
doc string to _validate_query and _validate_timestamp_fields
functions, related to the bugfix of bug 1280975.

Change-Id: I7c8564ce88d048e5b7d6d1e3c7109c5cb08e48f3
Merge "Adds doc string to query validate functions in V2 API"