Allow to flag a function for missing documentation

[floitsch]

It would be great to have a way to flag an element (class, function, ...) for missing documentation.
Ideally this should simply be a "request for more documentation" button.
Maybe we could allow the user to enter a question that they would like to see answered, as well.

Similarly "request more examples" (again with a button for what exactly they are missing).

That would require a server to collect the data, but would be extremely useful to help prioritize toitdocs.