[3.10] bpo-45680: Clarify documentation on ``GenericAlias`` objects (GH-29335) (GH-30688)

The documentation on ``GenericAlias`` objects implies at multiple points that
only container classes can define ``__class_getitem__``. This is misleading.
This PR proposes a rewrite of the documentation to clarify that non-container
classes can define ``__class_getitem__``, and to clarify what it means when a
non-container class is parameterized.

See also: initial discussion of issues with this piece of documentation in
GH-29308, and previous BPO issue [42280]().

Also improved references in glossary and typing docs. Fixed some links.

Co-authored-by: Erlend Egeberg Aasland <erlend.aasland@innova.no>
Co-authored-by: Ken Jin <28750310+Fidget-Spinner@users.noreply.github.com>
(cherry picked from commit 0eae9a2a2db6cc5a72535f61bb988cc417011640)


Co-authored-by: Alex Waygood <Alex.Waygood@Gmail.com>

Automerge-Triggered-By: GH:Fidget-Spinner

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