Every policy's documentation has a paragraph on what version of CMake
introduced it, how to set the policy, and whether CMake warns if the
policy is not set. The wording of this paragraph has diverged across
policies over time. Factor the paragraph out into a standard advice
document included by every policy.
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>
Alex Turbov