DOC: add behaviour change note justify this API choice

Author: tacaswell

doc/api/next_api_changes/behavior/22229-TAC.rst

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+AritistList proxies copy contents on iteration
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+When iterating over the contents of the the dynamically generated proxy lists
+for the Artist-type accessors (see :ref:`Behavioural API Changes 3.5 - Axes
+children combined`), a copy of the contents is made.  This ensure that artists
+can safely be added or removed from the Axes while iterating over their children.
+
+This is a departure from the expected behavior of mutable iterable data types
+in Python -- iterating over list while mutating it has surprising consequences
+and dictionaries will error if they change size during iteration.  In this case
+because all of the accessors are filtered views of the same underlying list, it
+is possible for seemingly unrelated changes, such as removing a Line, to affect
+the to affect the iteration over any of the other accessors, we have opted to
+make a copy of the relevant children before yielding them to the user.
+
+This change is also consistent with our plan to make these accessors immutable
+in Matplotlib 3.7.