Document exclude{min,max}.

The ZODB4 BTrees excludemin/excludemax arguments got implemented
in 3.3, but the corresponding changes to the BTrees interface
file got dropped on the floor.

NEWS.txt

@@ -98,6 +98,13 @@ but these work again too:
 - ``from ZODB.PersistentList import PersistentList``
 - ``from ZODB.PersistentMapping import PersistentMapping``
 
+BTrees
+------
+
+The BTrees interface file neglected to document the optional
+``excludemin`` and ``excludemax`` arguments to the ``keys()``, ``values()``
+and ``items()`` methods.  Appropriate changes were merged in from the
+ZODB4 BTrees interface file.
 
 fsIndex
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