[3.11] gh-106939: document ShareableList nul-strip quirk. (GH-107266) (#107270)

gh-106939: document ShareableList nul-strip quirk. (GH-107266)

* gh-106939: document ShareableList nul-strip quirk.
* Mention the `int` size constraint.
(cherry picked from commit 70dc00946938027d5a79bcb7b65038319040144e)

Co-authored-by: Gregory P. Smith <greg@krypto.org>