[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 30488fa)

Co-authored-by: Irit Katriel <[email protected]>

Author: miss-islington

Doc/library/gc.rst

@@ -50,6 +50,9 @@ The :mod:`gc` module provides the following functions:
    is run.  Not all items in some free lists may be freed due to the
    particular implementation, in particular :class:`float`.
 
+   The effect of calling ``gc.collect()`` while the interpreter is already
+   performing a collection is undefined.
+
 
 .. function:: set_debug(flags)