cmd/compile/internal/typecheck: update doc for the 1.18 export format
cmd/compile/internal/typecheck: update doc for the 1.18 export format

Update the documentation in iexport.go to use the word 'Constraint'
rather than 'Bound', and to account for recent changes to the export
format.

Change-Id: I83fbcd336d5f865af804bc8bef4f5d997cd9f325
Reviewed-on: https://go-review.googlesource.com/c/go/+/358547
Trust: Robert Findley <9179eee875d1ee628c37fb47d7e749e855ffd21f@google.com>
Trust: Dan Scales <7338a4f274e3bf1354488b9d1f344f4c9a69a8fb@google.com>
Run-TryBot: Robert Findley <9179eee875d1ee628c37fb47d7e749e855ffd21f@google.com>
TryBot-Result: Go Bot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Dan Scales <7338a4f274e3bf1354488b9d1f344f4c9a69a8fb@google.com>

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