Merge: Fallback doc

In order to fill the descriptions on the [overview page], add a way for a mproject or a mgroup to use the mdoc of other entities.

  [overview page]: http://nitlanguage.org/doc/stdlib/

Pull-Request: #954
Reviewed-by: Alexandre Terrasa <alexandre@moz-code.org>

diff --cc src/doc/doc_model.nit
Simple merge