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.
This is an old module from when VTK and other Kitware projects drove the
addition of commands and modules into CMake itself. Modern VTK doesn't
need this module and it can be ignored.
This module is kept around so that the ancient VTK versions which use it
are not broken by it. VTK itself stopped using the module in 2012 and
the last usage by an example in VTK was removed in 2020.
Fixes: #20591
Alex Turbov