[3.12] annotations: expand documentation on "simple" assignment targets (GH-120535) (#120556)

This behavior is rather surprising and it was not clearly specified.

(cherry picked from commit 9e0b11eb21930b7b8e4a396200a921e9985cfca4)

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