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.
The OLD behavior only removes a non-cache variable of the same
name in specific circumstances. The previous wording implied
that it would always occur.
Also add a note about the behavior compared to the analogous
CMP0077 policy, which affects the option() command in a similar
but subtly different way.
