add rustdoc for migrate

substrate/frame/staking/src/pallet/mod.rs

@@ -2181,6 +2181,13 @@ pub mod pallet {
 			Ok(())
 		}
 
+		/// Migrates permissionlessly a stash from locks to holds.
+		///
+		/// This removes the old lock on the stake and creates a hold on it atomically. If all
+		/// stake cannot be held, the best effort is made to hold as much as possible. The remaining
+		/// stake is removed from the ledger.
+		///
+		/// The fee is waived if the migration is successful.
 		#[pallet::call_index(30)]
 		#[pallet::weight(T::WeightInfo::migrate_currency())]
 		pub fn migrate_currency(