go/types: clarify the documentation for Func.Pkg

Change-Id: Ia695960ba652143f4a48c1ca3495a043097acbb7
Reviewed-on: https://go-review.googlesource.com/c/go/+/534298
TryBot-Result: Gopher Robot <[email protected]>
Reviewed-by: Robert Findley <[email protected]>
Reviewed-by: Alan Donovan <[email protected]>
Run-TryBot: Jonathan Amsterdam <[email protected]>

Author: jba

src/cmd/compile/internal/types2/object.go

@@ -406,6 +406,12 @@ func (obj *Func) Origin() *Func {
 	return obj
 }
 
+// Pkg returns the package to which the function belongs.
+//
+// The result is nil for methods of types in the Universe scope,
+// like [error.Error].
+func (obj *Func) Pkg() *Package { return obj.object.Pkg() }
+
 // hasPtrRecv reports whether the receiver is of the form *T for the given method obj.
 func (obj *Func) hasPtrRecv() bool {
 	// If a method's receiver type is set, use that as the source of truth for the receiver.