[3.8] bpo-40895: Update weakref documentation to remove old warnings (GH-20687) (GH-20792)

The doccumentation at https://docs.python.org/3.10/library/weakref.html cautions that the `WeakKeyDictionary` and `WeakValueDictionary` are susceptible to the problem of dictionary mutation during iteration.

These notes present the user with a problem that has no easy solution.

I dug into the implementation and found that fortunately, Antoine Pitrou already addressed this challenge (10 years ago!) by introducing an `_IterationGuard` context manager to the implementation, which delays mutation while an iteration is in progress.

I asked for confirmation and @pitrou agreed that these notes could be removed:
https://github.com/python/cpython/commit/c1baa601e2b558deb690edfdf334fceee3b03327GH-commitcomment-39514438.
(cherry picked from commit 1642c0ef750f96664a98cadb09301d492098d2fb)

Co-authored-by: Daniel Fortunov <asqui@users.noreply.github.com>

Automerge-Triggered-By: @pitrou

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