library: improve header file item comments, <STAT> api

library: improve header file item comments, <STAT> api

This patch just polishes the 'origin' comments for the
<STAT> header file. In particular those derived/unique
items (the 'SUM' guys) will now be properly explained.

[ in order to employ the 'derived from above' phrase ]
[ with their DELTA versions, all SUM items had to be ]
[ relocated (and some renamed). in turn, that had an ]
[ impact on many portions of the .c source file too. ]

Reference(s):
. summary calculations introduced
commit 2c86c4984a15c9ab912402838c6651f466a3d2ed
. origins added to header files
commit 96d59cbf46b3ff687bd29fad4708074a0e1cea14

Signed-off-by: Jim Warner <james.warner@comcast.net>