Added documentation for the new IsisPreferences options to specify the use of...

Added documentation for the new IsisPreferences options to specify the use of Embree/Bullet and also removed unnecessary checks for removed IsisPreferences keywords. Fixes #5184

git-svn-id: http://subversion.wr.usgs.gov/repos/prog/isis3/trunk@8249 41f8697f-d340-4b68-9986-7bafba869bb8