have the spec and implementations agree on how tuple reprs support optional fields

The implementation at https://github.com/ipld/go-ipld-prime/blob/e39d20bf18dcc21a50ef2005bae6de11e5c311e0/schema/gen/go/genStructReprTuple.go#L13 says they are allowed as trailing fields:

> // Optional fields for tuple representation are only allowed at the end, and contiguously.

However, the current spec at https://ipld.io/docs/schemas/features/representation-strategies/#struct-tuple-representation says:

> Optional or implicit fields are not possible with the tuple Struct representation strategy, all elements must be present.

For now, bindnode follows codegen; see https://github.com/ipld/go-ipld-prime/pull/368.

We should probably adjust the docs to agree with the codegen. cc @warpfork @rvagg