context: discourage use of basic types as keys in WithValue

Fixes #17302

Change-Id: I375d5d4f2714ff415542f4fe56a548e53c5e8ba6
Reviewed-on: https://go-review.googlesource.com/30134Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/context/context.go

@@ -443,7 +443,9 @@ func WithTimeout(parent Context, timeout time.Duration) (Context, CancelFunc) {
 // Use context Values only for request-scoped data that transits processes and
 // APIs, not for passing optional parameters to functions.
 //
-// The provided key must be comparable.
+// The provided key must be comparable and should not be
+// of type string or any other built-in type.
+// Users of WithValue should define their own types for keys.
 func WithValue(parent Context, key, val interface{}) Context {
 	if key == nil {
 		panic("nil key")