rustdoc: show struct field docs when inlined

Some minor changes to the compiler to expose this information. Very
inconvenient since struct fields aren't an item.
There seems to already be a `encode_struct_fields` method which seems like this call to `encode_attributes` should be placed inside (rather than having a separate section). Did you try that and end up having it fall short of what should happen? It may also allow hooking into the existing `load_attrs` function instead of having to redefined the `decode_*` variants.