PROG - Added additional documentation to the Spice class header file to...

PROG - Added additional documentation to the Spice class header file to explain the corrections for NAIF observer/target order and light time correction to the target body surface.  References #909, #1136 and #1223

git-svn-id: http://subversion.wr.usgs.gov/repos/prog/isis3/trunk@4910 41f8697f-d340-4b68-9986-7bafba869bb8