[spec/statement] Clarify foreach restrictions

[ntrel]

Fixes #22910.

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[dkorpel]

Does 'it' refer to the operation or the enclosing function? The latter makes more sense to me, but the wording implies the former.

[ntrel]

@dkorpel I think if the operation is allowed in @safe code, it should be implementation-defined iff it happens in @system code which doesn't do anything unsafe (or it does something unsafe that is unrelated to the operation).