updated RelayerVersion documentation to support multiple running rela…

…yer versions (see paritytech/parity-bridges-common#2891 for request)

bridges/primitives/runtime/src/lib.rs

@@ -542,7 +542,14 @@ where
 )]
 pub struct RelayerVersion {
 	/// Version that is increased manually. Increase it every time when something that
-	/// breaks the relayer and can't be detected with tests is changed.
+	/// may break already running relayers is changed. This is an always increasing
+	/// value and relayers that are using older version will immediately shutdown, while
+	/// relayers using newer value will keep running until runtime is upgraded to that
+	/// new version.
+	///
+	/// **IMPORTANT**: it must be changed every time [`Self::auto`] value is changed.
+	/// It also msut be changed if [`Self::auto`] stays the same and there's some breaking
+	/// change that is not detected by tests.
 	///
 	/// Every change to this field shall result in building and deploying a new relayer
 	/// version.
@@ -551,6 +558,8 @@ pub struct RelayerVersion {
 	/// tests righteously fail, meaning that something has been changed in pallet/runtime
 	/// configuration that breaks the existing relayer.
 	///
+	/// **IMPORTANT**: do not
+	///
 	/// Every change to this field shall result in building and deploying a new relayer
 	/// version.
 	pub auto: H256,