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.
Previously we always used content guarded by `$<LINK_ONLY:...>`
in `LINK_LIBRARIES`, even when evaluating for non-linking usage
requirements. Add a policy to honor `LINK_ONLY` in `LINK_LIBRARIES`
the same way we already do in `INTERFACE_LINK_LIBRARIES`.
Alex Turbov