Improvement: Expand background details on PDV in the LTD

Improvement: Expand background details on PDV in the LTD

Expand description of Packet Delay Variation to capture
the additional details which will be important to measurement
design. Measurmeent devices may be implementing the RFC 5481
PDV metric for the firt time, so som general details will
be useful to include in the LTD. Addresses MT's comments.

JIRA: VSPERF-73

Change-Id: I60524969f72bb00519e8070ae51e219d857c554e
Signed-off-by: Al Morton <acmorton@att.com>
Reviewed-by: Maryam Tahhan <maryam.tahhan@intel.com>