cmd/compile/internal/syntax: fix scanner.{op,prec} docs
cmd/compile/internal/syntax: fix scanner.{op,prec} docs

These fields are also valid for _Star.

Change-Id: I2582000ac1737c41416e69f91404f7cac279d718
Reviewed-on: https://go-review.googlesource.com/c/go/+/395916
Trust: Matthew Dempsky <e56ac2c500064424afac424d4b5e53bd0570a731@google.com>
Reviewed-by: Robert Griesemer <a6752097faebf1f02d80107d77a9be009089afe3@golang.org>

golang/go,golang/go,golang/go,CAFxX/go,CAFxX/go,CAFxX/go,CAFxX/go,nawawi/go,golang/go,nawawi/go,nawawi/go,nawawi/go,nawawi/go,golang/go,CAFxX/go,nawawi/go,CAFxX/go,CAFxX/go,golang/go,nawawi/go,nawawi/go,golang/go,golang/go,CAFxX/go