[3.13] gh-141004: Document PyType_SUPPORTS_WEAKREFS (GH-141408)

(cherry picked from commit 9cd5427)

Co-authored-by: Peter Bierma <zintensitydev@gmail.com>
Co-authored-by: Stan Ulbrych <89152624+StanFromIreland@users.noreply.github.com>

Author: ZeroIntensity