added Mesh.calc_tessface(), needed to update mesh tessface after bmesh edits.

also add py api BMDeformVert.clear()

doc/python_api/rst/include__bmesh.rst

@@ -75,6 +75,16 @@ its good practice to call :class:`bmesh.types.BMesh.free` which will remove all
 further access.
 
 
+EditMode Tessellation
+^^^^^^^^^^^^^^^^^^^^^
+
+When writing scripts that operate on editmode data you will normally want to re-calculate the tessellation after
+running the  script, this needs to be called explicitly.
+
+The BMesh its self does not store the triangulated faces, they are stored in the :class:`bpy.types.Mesh`,
+to refresh tessellation faces call :class:`bpy.types.Mesh.calc_tessface`.
+
+
 CustomData Access
 -----------------