The caller needs to know the protocol. The API writer has to document something like widget-frob may raise a widget-error condition, in which case the foo and bar restarts are available which proceed in such and such ways.

The behavior is also discoverable interactively. You play with widget-frob at the REPL, and hit a widget-error condition. This recurses into a debug prompt where you are told about the available restarts, which have docstring-like descriptions.