note scoping for AggregateInstanceExtraSpecsFilter

Havana release fixed a bug where this filter counldn't be used
with other filters due to scoping issue. This doc patch updates
to note that all is fine now.

Change-Id: Ic9f832c3ad09a5b5d9237b77952aa59ff83121dd
Closes-Bug: 1198310
Merge "note scoping for AggregateInstanceExtraSpecsFilter"