I agree it's not a good idea to be too much specific about this. The
patch attached adds the following footnote to the 'is' operator:

Due to automatic garbage-collection, free lists, and the dynamic nature
of descriptors, you may notice unusual behaviour in certain combinations
of :keyword:`is` operator, like those involving identity comparisons
between class and instancemethods, and free instances in the same
expression. Check their docs for more info.