example of Physics Constraints module :)

+ some small fixes in other docs.

A topic for later(soon?), I think we should name the modules according to the rest of Blender modules. So instead of:
Game Engine bge.logic Module

We would have it:
Game Logic (bge.logic)
...

doc/python_api/examples/blf.py

@@ -5,7 +5,6 @@ Blender Game Engine example of using the blf module. For this module to work we
 need to use the OpenGL wrapper :class:`~bgl` as well.
 """
 # import game engine modules
-import bge
 from bge import render
 from bge import logic
 # import stand alone modules