reflect the Version 9.3.1 change from sstats structures to sstats
records, with sstats times represented as time objects and sstats
bytes and counts represented as exact integers; also updated the
sstats-difference description to reflect that it no longer coerces
negative differences to zero. added a corresponding release note.
system.stex,
release_notes.stex
original commit: cd93f130c2a911d67bc19b75da0f205d50b8f6ff
