net/textproto: turn an ancient DoS BUG annotation into a comment

Actually fixing this "bug" would be weird, since io.LimitReader already
does what we need, as demonstrated by net/http's use.

Thanks to @davidfstr for pointing this out.

Change-Id: If707bcc698d1666a369b39ddfa9770685fbe3879
Reviewed-on: https://go-review.googlesource.com/1579
Reviewed-by: Rob Pike <r@golang.org>