cleanup runtime docs

tokio/src/runtime/local_runtime/runtime.rs

@@ -14,9 +14,14 @@ use std::time::Duration;
 /// This runtime is capable of driving tasks which are not `Send + Sync` without the use of a
 /// `LocalSet`, and thus supports `spawn_local` without the need for a LocalSet context.
 ///
+/// This runtime cannot be moved between threads or driven from different threads.
+///
 /// This runtime is incompatible with LocalSet. You should not attempt to drive a LocalSet within a
 /// LocalRuntime.
 ///
+/// Currently, this runtime supports one flavor, which is internally identical to current_thread,
+/// save for the aforementioned differences related to spawn_local.
+///
 /// For more general information on how to use runtimes, see the [module] docs.
 ///
 /// [runtime]: crate::runtime::Runtime