capnp: clarify that clients are safe to use from multiple goroutines
capnp: clarify that clients are safe to use from multiple goroutines

zombiezen/go-capnproto,hodduc/go-capnproto2,zombiezen/go-capnproto2,zombiezen/go-capnproto2,hodduc/go-capnproto2,zombiezen/go-capnproto