[3.11] gh-49174: document that the effect of calling gc.collect() during a collection is undefined (GH-104699) (#104703)

gh-49174: document that the effect of calling gc.collect() during a collection is undefined (GH-104699)
(cherry picked from commit 30488fa22a8f63753192ae7f1d01665857764e45)

Co-authored-by: Irit Katriel <1055913+iritkatriel@users.noreply.github.com>