define_list_builder_accessors contrives to hide documentation

Because define_list_builder_accessors is not a derive macro, it does not receive the struct definition. So the docs for the generated accessors are formulaic. That part is kind of OK, but the actual docs for the field end up only in the docs for the main struct field. Which is private.

So overall, the docs are hidden.

I think the fix is to move the public docs from the list struct fields to into the macro call. I experimented with this and it seems to work.