Add additional info to clear up confusion about XDCR filtering

There have been some recent XDCR filtering questions being asked that indicated some confusion about the feature.  So, would like additional info to be added to the existing documentation to explain that the passage of time will not cause the XDCR filter to be applied.

In https://docs.couchbase.com/server/current/learn/clusters-and-availability/xdcr-filtering.html#configure-xdcr-filtering 

After the first sentence:

Understanding XDCR Advanced Filtering
XDCR filtering allows a document to be included in or excluded from a filtered replication, based on the document’s fields and values.

Add:

Please note that XDCR applies the filter during replication processing, as the data is passed through the replication pipeline.  Therefore, the filter evaluation is done only when there is a change to the document (when the document is created or updated), or if the replication is being restarted for all documents. So, filtering using a time value in a document, like “expiration_time”, and expecting that the document will be replicated when the expiration_time becomes less than some value will not work if you are expecting that the replication will be triggered by passage of time. The passage of time will not cause the filter to be applied.