[poem-openapi] Use doc comment to generate Union variant documentation

[Zstorm999]

Description of the feature

It would be nice to be able to document Unions directly from doc comments on the variant. Currently this does not work, and the documentation is extracted from the doc comment on the struct contained in the variant instead.

This behaviour does not seem consistent with what happens inside Objects, where doc comments on each field are used as the documentation in the generated api.

Feature example

#[derive(Union)]
enum MyEnum {
    /// variant A doc comment    <-- this is currently ignored
    A(Struct_A),
    /// variant B doc comment    <-- this is also currently ignored
    B(Struct_B),
}