[docs] [issue17546] Document the circumstances where the locals() dict get updated

[anatoly techtonik]

anatoly techtonik added the comment:

> ... cannot be relied upon to either happen or not happen...

IMHO this phrase is from Advanced English course.

> The original included the caution against modifying it, and I think it is
valid because of the inconsistent behavior.
> Perhaps it could be weakened to "it is not a good idea to modify"?

Ambiguity already adds fear and uncertainty. Just explaining the
consequences would be good style for the technical doc.

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