Add a document describing all of the uses of sv_gametype.

Useful for perhaps refactoring gametype functionality in the future.

git-svn-id: http://odamex.net/svn/root/trunk@3721 b0bcef46-2027-0410-bd56-ab726e9a1d04
-Fix a railgun aiming bug that occurred if the rail tracer hit a sky-wall hack.

git-svn-id: http://odamex.net/svn/root/trunk@3722 b0bcef46-2027-0410-bd56-ab726e9a1d04