[3.11] Enhance TypedDict docs around required/optional keys (GH-109547) (#109983)

As discussed in comments to GH-109544, the semantics of this attribute
are somewhat confusing. Add a note explaining its limitations and
steering users towards __required_keys__ and __optional_keys__ instead.
(cherry picked from commit f49958c886a2f2608f1008186d588efc2a98b445)

Co-authored-by: Jelle Zijlstra <jelle.zijlstra@gmail.com>