The auxiliary field could be better documented as it appears in a lot of the operands.
This CL documents and points the user to the code for further information.
Change-Id: I34a73af488358c9799e286f3b5ee4b08639ce608
Reviewed-on: https://go-review.googlesource.com/c/go/+/613615
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: Keith Randall <khr@google.com>