context: mention standard library version in package comment

Fixes golang/go#22322

Change-Id: I1f0c62ce1c247b583de51faef8722d07e627b441
Reviewed-on: https://go-review.googlesource.com/72570Reviewed-by: Ian Lance Taylor <iant@golang.org>

context/context.go

@@ -5,6 +5,8 @@
 // Package context defines the Context type, which carries deadlines,
 // cancelation signals, and other request-scoped values across API boundaries
 // and between processes.
+// As of Go 1.7 this package is available in the standard library under the
+// name context.  https://golang.org/pkg/context.
 //
 // Incoming requests to a server should create a Context, and outgoing calls to
 // servers should accept a Context. The chain of function calls between must