[3.12] gh-123242: Note that type.__annotations__ may not exist (GH-124557) (#124562)

Closes GH-123242. The real criterion is that the attribute does not
exist on heap types, but I don't think we should discuss heap vs.
static types in the language reference.
(cherry picked from commit 99b23c64de301c9e77add6b0d8e60118ef807840)