Include all docstrings of a module

[RobertDober]

This usecase comes from the following pattern

<%= xtra.moduledoc "MyModule", ... %>
<%= xtra.functiondoc :all, module: "MyModule", ... %>

it would be nice to condense this repeating pattern

e.g.

<%= xtra.moduledoc "MyModule", include: ["functiondocs", "macrodocs"], ... %>