go/ast: better documentation for CallExpr node

Fixes #17222.

Change-Id: Iffffc8cbb8627d06afa9066246b68fa2da4600e3
Reviewed-on: https://go-review.googlesource.com/29810
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

src/go/ast/ast.go

@@ -317,7 +317,7 @@ type (
 		Fun      Expr      // function expression
 		Lparen   token.Pos // position of "("
 		Args     []Expr    // function arguments; or nil
-		Ellipsis token.Pos // position of "...", if any
+		Ellipsis token.Pos // position of "..." (token.NoPos if there is no "...")
 		Rparen   token.Pos // position of ")"
 	}