Add support for function specs with multiple clauses

[dumbbell]

Here is an example:

-spec f(foo) -> bar;
       (bar) -> baz.

Before this patch, rendering the docs with rebar3 would crash with the following exception:

edoc: error in layout 'edown_layout': {'EXIT',
                                 {{case_clause,
                                   [{xmlElement,typespec,[],[],
                                     {xmlNamespace,[],[]},
                                     [{function,27},{functions,2},{module,1}],
				     ...

Fixes uwiger/edown#23.