spec: describe constraint parsing ambiguity and work-around more precisely

The new description matches the implementation (CL 370774).

Also, in the section on type constraints, use "defines" instead of
"determines" because the constraint interface defines the type set
which is precisely the set of acceptable type arguments.

For #49482.

Change-Id: I6f30f49100e8ba8bec0a0f1b450f88cae54312eb
Reviewed-on: https://go-review.googlesource.com/c/go/+/372874


Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>