API doc: Gotcha's: Add section about abusing RNA properties callbacks.

Especially with new undo/redo it is even less recommended to perform
complex operations in those callbacks, they should remain as fast and
localized as possible.

Also updated the section about undo/redo a bit.

doc/python_api/rst/info_gotcha.rst

@@ -677,7 +677,8 @@ Here are some general hints to avoid running into these problems:
 Undo/Redo
 ---------
 
-Undo invalidates all :class:`bpy.types.ID` instances (Object, Scene, Mesh, Light, etc.).
+For safety, you should assume that undo and redo always invalidates all :class:`bpy.types.ID` 
+instances (Object, Scene, Mesh, Light, etc.), as weel obviously as all of their sub-data.
 
 This example shows how you can tell undo changes the memory locations:
 
@@ -686,7 +687,7 @@ This example shows how you can tell undo changes the memory locations:
    >>> hash(bpy.context.object)
    -9223372036849950810
 
-Move the active object, then undo:
+Delete the active object, then undo:
 
    >>> hash(bpy.context.object)
    -9223372036849951740
@@ -695,6 +696,16 @@ As suggested above, simply not holding references to data when Blender is used
 interactively by the user is the only way to make sure that the script doesn't become unstable.
 
 
+.. note::
+
+   Modern undo/redo system does not systematically invalidate all pointers anymore.
+   Some data (in fact, most data, in typical cases), which were detected as unchanged for a
+   particular history step, may remain unchanged and hence their pointers may remain valid.
+   
+   Be aware that if you want to take advantage of this behavior for some reason, there is no
+   guarantee of any kind that it will be safe and consistent. Use it at your own risk.
+
+
 Undo & Library Data
 ^^^^^^^^^^^^^^^^^^^
 
@@ -712,6 +723,17 @@ So it's best to consider modifying library data an advanced usage of the API
 and only to use it when you know what you're doing.
 
 
+Abusing RNA property callbacks
+------------------------------
+
+Python-defined RNA properties can have custom callbacks. Trying to perform complex operations
+from there, like calling an operator, may work, but is not officialy recommended nor supported.
+
+Main reason is that those callback should be very fast, but additionally, it may for example
+create issues with undo/redo system (most operators store an history step, and editing an RNA
+property does so as well), trigger infinite update loops, and so on.
+
+
 Edit-Mode / Memory Access
 -------------------------