docgen: Custom `attrdef` don't seem to appear in generated documentation.

[AtomicSynth]

Would be great if standalone attrdef appeared in generated documentation.
This is what I currently have and it's missing.

<*
 Use on a struct or field to bypass requiring name.
*>
attrdef @Arg = @tag("ion_arg", true);

<*
 Use only for struct members that are slices, this way you won't have to wrap
 values in () and just type them in as normal arguments
*>
attrdef @Flat = @tag("ion_flat", true);

<*
 Tell Ion to use alternative name for type/field
*>
attrdef @Name(name) = @tag("ion_name", name);