[3.12] GH-95088: Clarify rules for parsing an item key for format strings (GH-103779) (#105419)

GH-95088: Clarify rules for parsing an item key for format strings (GH-103779)
(cherry picked from commit 3e7316d7e8969febb56fbc7416d483b073bd1702)

Co-authored-by: achhina <amanschhina@gmail.com>
Co-authored-by: Łukasz Langa <lukasz@langa.pl>
Co-authored-by: Jelle Zijlstra <jelle.zijlstra@gmail.com>