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 REAL_PATH would collapse paths before resolving any symlinks
so if `..` crossed a symlink the output from `REAL_PATH` would be wrong.
It looked like REAL_PATH did this by mistake since it was a side-effect
of ensuring we had an absolute path before resolving symlinks.