Add a note in output documentation for fields with refs to opaques

Add a note in output documentation for fields with refs to opaques

Because we (probably shouldn't, but do) map "opaque" structs with
an `is_owned` field instead of having a dedicated reference type,
fields which are a `Deref<Struct>` are simply mapped as
`OpaqueMappedStruct`. This is confusing for downstream `lightning.h`
consumers as they cannot differentiate between a field that expects
to own a `Struct` and one which expects to own a reference to it.

This is worked around here by adding documentation which can be
matched on, though ultimately structs should move towards full
ownership.