go/ast: clarify when Ident.Obj is nil

Fixes #48141

Change-Id: Id20b7801d31456ffd74301ed0fd84788b8982fb1
Reviewed-on: https://go-review.googlesource.com/c/go/+/347530
Trust: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Robert Findley <rfindley@google.com>
TryBot-Result: Go Bot <gobot@golang.org>

src/go/ast/ast.go

@@ -290,7 +290,7 @@ type (
 	Ident struct {
 		NamePos token.Pos // identifier position
 		Name    string    // identifier name
-		Obj     *Object   // denoted object; or nil
+		Obj     *Object   // denoted object; or nil if the referenced declaration is not found in the parsed scope
 	}
 
 	// An Ellipsis node stands for the "..." type in a