syscall/js: document ValueOf() panic

ValueOf() panics if x is not one of the expected types.

Change-Id: I1105e46bd09a5ab13c162b77c1c50cc45bce27a2
GitHub-Last-Rev: 34a88ce8206954d94f0a884ab7f6494116c54a2d
GitHub-Pull-Request: golang/go#28846
Reviewed-on: https://go-review.googlesource.com/c/150138Reviewed-by: Richard Musiol <neelance@gmail.com>

src/syscall/js/js.go

@@ -114,6 +114,8 @@ func Global() Value {
 //  | string                 | string                 |
 //  | []interface{}          | new array              |
 //  | map[string]interface{} | new object             |
+//
+// Panics if x is not one of the expected types.
 func ValueOf(x interface{}) Value {
 	switch x := x.(type) {
 	case Value: // should precede Wrapper to avoid a loop