BGE: Add missing documentation and attribute constraint_type for ConstraintWrapper

1. Add attribute to get the constraint type.
2. Add missing documentation for getParent, setParam, constraint_id in bge.types.KX_ConstraintWrapper.rst.
3. Add missing documentation for GENERIC_6DOF_CONSTRAINT and flag bit in bge.constraints.rst.
4. Fix typo in CcdPhysicsEnvironment.cpp

Reviewers: moguri

Reviewed By: moguri

Differential Revision: https://developer.blender.org/D654

doc/python_api/rst/bge.constraints.rst

@@ -28,6 +28,7 @@ Physics Constraints (bge.constraints)
    - :class:`ANGULAR_CONSTRAINT`
    - :class:`CONETWIST_CONSTRAINT`
    - :class:`VEHICLE_CONSTRAINT`
+   - :class:`GENERIC_6DOF_CONSTRAINT`
 
    :type constrainttype: int
 
@@ -49,7 +50,7 @@ Physics Constraints (bge.constraints)
    :arg axisZ: Z axis
    :type axisZ: float
 
-   :arg flag: .. to do
+   :arg flag: 128 to disable collision between linked bodies
    :type flag: int
 
 .. attribute:: error
@@ -368,3 +369,10 @@ Physics Constraints (bge.constraints)
       Constraint type to be used with function :class:`createConstraint`
 
    .. to do
+
+.. data:: GENERIC_6DOF_CONSTRAINT
+
+   .. note::
+      Constraint type to be used with function :class:`createConstraint`
+
+   .. to do