BGE documentation updates

Mostly added missing :arg: clauses -- these caused the :type:-clauses
that were already in the RST files to be hidden from the output HTML.

I also fixed some argument types from 'list' to 'Vector' where applicable
and corrected some obvious mistakes.

doc/python_api/rst/bge.logic.rst

@@ -258,7 +258,8 @@ General functions
 
    Sets the world gravity.
    
-   :type gravity: list [fx, fy, fz]
+   :arg gravity: gravity vector
+   :type gravity: Vector((fx, fy, fz))
 
 .. function:: getSpectrum()