rpcsrv: return error on invalid proof from verifyProof

This change makes code incompatible with C# node,
because currently no error is returned on invalid proof.
According to proposal:
https://github.com/neo-project/proposals/pull/156
Also adding `verifyProof` descpiption in docs/rpc.md.

Signed-off-by: Tatiana Nesterenko <tatiana@nspcc.io>

docs/rpc.md

@@ -196,6 +196,11 @@ enabled in the server's protocol configuration.
 ##### `getnep11transfers` and `getnep17transfers`
 `transfernotifyindex` is not tracked by NeoGo, thus this field is always zero.
 
+##### `verifyProof`
+
+NeoGo can generate an error in response to an invalid proof, unlike
+the error-free C# implementation.
+
 ### Unsupported methods
 
 Methods listed below are not going to be supported for various reasons

pkg/services/rpcsrv/server.go

@@ -1531,9 +1531,10 @@ func (s *Server) verifyProof(ps params.Params) (any, *neorpc.Error) {
 	}
 	vp := new(result.VerifyProof)
 	val, ok := mpt.VerifyProof(root, p.Key, p.Proof)
-	if ok {
-		vp.Value = val
+	if !ok {
+		return nil, neorpc.ErrInvalidProof
 	}
+	vp.Value = val
 	return vp, nil
 }