manifest-format: clarify <copyfile> & <linkfile> restrictions

While we don't (yet) explicitly enforce all of these, make sure
we document the expected behavior so we can all agree on it.

Bug: https://crbug.com/gerrit/11218
Change-Id: Ife8298702fa445ac055ef43c6d62706a9cb199ce
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/232893
Reviewed-by: David Pursehouse <dpursehouse@collab.net>
Tested-by: Mike Frysinger <vapier@google.com>