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.
Extend commit eb35d8884b (find_package: Use PackageName_ROOT variables
as search prefixes, 2018-03-15, v3.12.0-rc1~349^2) to also check
upper-case `<PACKAGENAME>_ROOT` variables. Add policy `CMP0144` to
enable the behavior in a compatible way.
Fixes: #24403