tests/cpydiff: Add a note about risk of resizing memoryview targets.

This a stop-gap until there is a proper fix for this.

This work was funded through GitHub Sponsors.
Signed-off-by: Angus Gratton <angus@redyak.com.au>

tests/cpydiff/types_memoryview_invalid.py

+"""
+categories: Types,memoryview
+description: memoryview can become invalid if its target is resized
+cause: CPython prevents a ``bytearray`` or ``io.bytesIO`` object from changing size while there is a ``memoryview`` object that references it. MicroPython requires the programmer to manually ensure that an object is not resized while any ``memoryview`` references it.
+
+In the worst case scenario, resizing an object which is the target of a memoryview can cause the memoryview(s) to reference invalid freed memory (a use-after-free bug) and corrupt the MicroPython runtime.
+workaround: Do not change the size of any ``bytearray`` or ``io.bytesIO`` object that has a ``memoryview`` assigned to it.
+"""
+b = bytearray(b"abcdefg")
+m = memoryview(b)
+b.extend(b"hijklmnop")
+print(b, bytes(m))