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.
Only generate a graph dependency between a custom command and
a target when the custom command queries for the file path
of an artifact of the target.
This makes generator expressions such as `TARGET_FILE_DIR`
behave the same way as `TARGET_PROPERTY` which never generated
a graph dependency.
Brad King