Rename OffchainWorker internal method. (#4267)

* Rename generate_extrinsics to offchain_worker

* Some doc improvements.

substrate/primitives/sr-primitives/src/traits.rs

@@ -346,18 +346,22 @@ pub trait OnInitialize<BlockNumber> {
 /// Off-chain computation trait.
 ///
 /// Implementing this trait on a module allows you to perform long-running tasks
-/// that make validators generate extrinsics (either transactions or inherents)
-/// with the results of those long-running computations.
+/// that make (by default) validators generate transactions that feed results
+/// of those long-running computations back on chain.
 ///
 /// NOTE: This function runs off-chain, so it can access the block state,
-/// but cannot preform any alterations.
+/// but cannot preform any alterations. More specifically alterations are
+/// not forbidden, but they are not persisted in any way after the worker
+/// has finished.
 #[impl_for_tuples(30)]
 pub trait OffchainWorker<BlockNumber> {
-	/// This function is being called on every block.
+	/// This function is being called after every block import (when fully synced).
 	///
-	/// Implement this and use special `extern`s to generate transactions or inherents.
+	/// Implement this and use any of the `Offchain` `sp_io` set of APIs
+	/// to perform offchain computations, calls and submit transactions
+	/// with results to trigger any on-chain changes.
 	/// Any state alterations are lost and are not persisted.
-	fn generate_extrinsics(_n: BlockNumber) {}
+	fn offchain_worker(_n: BlockNumber) {}
 }
 
 /// Abstraction around hashing