Improve the documentation of the dune policy DP_DEFAULT_INCLUDE_DIRS

With !1355 (merged) we have introduced the first dune policy into the buildsystem. It allows to change the behavior in a backward compatible way and each module can decide whether to use the OLD or the NEW buildsystem behavior. If a new policy is detected in the buildsystem and a module has not yet reacted on that, it will display a warning. This warning dow only contain a minimal set of information to guide the user what to do. This information might not be enough. The detailed instructions what to change is either give in the corresponding MR introducing the dune policy, or in the buildsystem documentation. This MR improves the buildsystem documentation for the new dune policy DP_DEFAULT_INCLUDE_DIRS