context: add docs to ExampleWithValue

Change-Id: I3a83c63f4db2e46fd96f373378a429896e93f9d1
Reviewed-on: https://go-review.googlesource.com/48861
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/context/example_test.go

@@ -93,6 +93,8 @@ func ExampleWithTimeout() {
 	// context deadline exceeded
 }
 
+// This example demonstrates how a value can be passed to the context
+// and also how to retrieve it if it exists.
 func ExampleWithValue() {
 	type favContextKey string