spec: clarify struct field and array element comparison order

[ferrmin]

Fixes golang#8606.

Change-Id: I64b13b2ed61ecae4641264deb47c9f7653a80356
Reviewed-on: https://go-review.googlesource.com/c/go/+/449536
Reviewed-by: Matthew Dempsky mdempsky@google.com
Reviewed-by: Ian Lance Taylor iant@google.com
Reviewed-by: Robert Griesemer gri@google.com
Auto-Submit: Robert Griesemer gri@google.com
Run-TryBot: Robert Griesemer gri@google.com
TryBot-Result: Gopher Robot gobot@golang.org

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