DDSIRTP2 — Clarify GAP usage

Source:
Real-Time Innovations, Inc. (Ken Brophy, ken@rti.com)
Summary:
Clarify that GAPs should be used only when a sample is not meaningful and not sent in the first place (e.g. filtered out).
Resolution:
Clarify GAP usage.

Revised Text:
Revise 8.3.7.4.5:

The RTPS Writer sends the Gap message to the RTPS Reader to communicate that certain sequence numbers are no longer relevant. This is typically caused by the KEEP_LAST setting of the History QoS on the corresponding DDS DataWriter. This is typically caused by Writer-side filtering of the sample (content-filtered topics, time-based filtering). In this scenario, new data-values may replace the old values of the data-objects that were represented by the sequence numbers that appear as irrelevant in the GAP.

see above