design: modify Clip godoc not to mention seeking during Frames calls
design: modify Clip godoc not to mention seeking during Frames calls

Change-Id: If9cb647909498866a9746aa6aea005b40d772587
Reviewed-on: https://go-review.googlesource.com/24869
Reviewed-by: Jaana Burcu Dogan <347c19a639ce752a783b4ea325d6539ab4b4a262@google.com>

golang/proposal,golang/proposal