[#92] client: Add docs about status returns

Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>

client/client.go

@@ -14,6 +14,16 @@ import (
 // Using the Client that has been created with new(Client)
 // expression (or just declaring a Client variable) is unsafe
 // and can lead to panic.
+//
+// Each method which produces a NeoFS API call may return a server response.
+// Status responses are returned in the result structure, and can be cast
+// to built-in error instance (or in the returned error if the client is
+// configured accordingly). Certain statuses can be checked using `apistatus`
+// and standard `errors` packages.
+// All possible responses are documented in methods, however, some may be
+// returned from all of them (pay attention to the presence of the pointer sign):
+//  - *apistatus.ServerInternal on internal server error;
+//  - *apistatus.SuccessDefaultV2 on default success.
 type Client struct {
 	raw *client.Client