Link to the original struct in `Proxy` doc

[ilchu]

I think we could add link to the original struct in docs, because we would have more than one proxy objects. Originally posted by @Erigara in https://github.com/hyperledger/iroha/pull/2602#discussion_r947701400

Determine if it's going to be needed in the final design, as proxies may end up being not exposed to the user at all and thus also not requiring any documentation.