Added tracked parachains filter (#1432)

* added trackerd parachains filter + changed some docs

* fix compilation

bridges/modules/grandpa/src/lib.rs

@@ -93,6 +93,8 @@ pub mod pallet {
 		/// The setting is there to prevent growing the on-chain state indefinitely. Note
 		/// the setting does not relate to block numbers - we will simply keep as much items
 		/// in the storage, so it doesn't guarantee any fixed timeframe for finality headers.
+		///
+		/// Incautious change of this constant may lead to orphan entries in the runtime storage.
 		#[pallet::constant]
 		type HeadersToKeep: Get<u32>;