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 <rfindley@google.com>
Trust: Dan Scales <danscales@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Dan Scales <danscales@google.com>
diff --git a/src/cmd/compile/internal/typecheck/iexport.go b/src/cmd/compile/internal/typecheck/iexport.go
index 6057000..f685851 100644
--- a/src/cmd/compile/internal/typecheck/iexport.go
+++ b/src/cmd/compile/internal/typecheck/iexport.go
@@ -97,9 +97,10 @@
 //
 //     // "Automatic" declaration of each typeparam
 //     type TypeParam struct {
-//         Tag  byte // 'P'
-//         Pos  Pos
-//         Bound typeOff
+//         Tag        byte // 'P'
+//         Pos        Pos
+//         Implicit   bool
+//         Constraint typeOff
 //     }
 //
 // typeOff means a uvarint that either indicates a predeclared type,
@@ -108,7 +109,7 @@
 // types list (see predeclared in bexport.go for order). Otherwise,
 // subtracting predeclReserved yields the offset of a type descriptor.
 //
-// Value means a type and type-specific value. See
+// Value means a type, kind, and type-specific value. See
 // (*exportWriter).value for details.
 //
 //