Update Client::transaction docs with ressources that show how to return custom error types

[d4h0]

This adds two links to the documentation of Client::transaction that show how to return custom error types from transactions (which was pretty unclear to me).

Feel free to change however you like (English isn't my native language).

[edgedb-cla[bot]]

All commit authors signed the Contributor License Agreement.

[tailhook]

Thanks!