allow documenting a struct

[t-bltg]

Fix the following issues with docstrings:

using ObjectOriented

"""
Documented struct fancy docstring
"""
@oodef struct DocumentedStruct
    a :: Int
    """
    Constructor docstring
    """
    function new(x)
        @mk a = 2x
    end
end

ERROR: LoadError: unrecognised statement in DocumentedStruct definition: Constructor docstring

# and

ERROR: LoadError: cannot document the following expression:

'@oodef' not documentable. See 'Base.@__doc__' docs for details.

[thautwarm]

Looks good