Docs: Clarify CMAKE_PARENT_LIST_FILE (#14194)

Fix the wording of the documentation for CMAKE_PARENT_LIST_FILE to
clarify that it only applies to the include stack in the current
directory.  Specify exactly what commands can include files instead of
saying "somehow invoke".
Merge topic 'doc-CMAKE_PARENT_LIST_FILE' into next

b62d440 Docs: Clarify CMAKE_PARENT_LIST_FILE (#14194)