improve documentation of runtime. there was no mention of types.

R=rsc
CC=golang-dev
https://golang.org/cl/157042

src/pkg/runtime/extern.go

@@ -4,7 +4,9 @@
 
 /*
 	The runtime package contains operations that interact with Go's runtime system,
-	such as functions to control goroutines.
+	such as functions to control goroutines. It also includes the low-level type information
+	used by the reflect package; see reflect's documentation for the programmable
+	interface to the run-time type system.
 */
 package runtime