Help: Normalize and consolidate standard policy advice paragraph

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.

Also revise the wording of the paragraph to:

• Avoid implying that a policy must always be set explicitly.
• Mention that cmake_minimum_required can set policies too.
• Drop the current CMake version since it does not affect the behavior.