Documenting the syntax in BNF

git-svn-id: http://svn.code.sf.net/p/angelscript/code@2066 404ce1b2-830e-0410-a2e2-b09542c77caf
Added ShutDownAndRelease to explicitly tell when the engine should be shut down, compared to simply releasing the reference

git-svn-id: http://svn.code.sf.net/p/angelscript/code@2067 404ce1b2-830e-0410-a2e2-b09542c77caf