Help: Document CMP0124 behavior on already-set variables

Improve the documentation from commit 46896d98bb (foreach(): loop
variables are only available in the loop scope, 2021-04-25,
v3.21.0-rc1~245^2) to follow policy documentation convention.

Fixes: #25224
Inspired-by: Marius Messerschmidt <marius.messerschmidt@googlemail.com>