all: reword comments to state behavior directly

Comments (and one test comment) now describe the rules on their own terms
instead of by comparison to another downloader. No behavior change.

main_test.go

@@ -147,7 +147,7 @@ func TestGatherURIsGrouping(t *testing.T) {
 // TestHTTPConnDefault locks the parallelism default: a bare `got URL` must use
 // several connections, not one. With -x unset, --split drives the per-host
 // connection count (capped at the -x ceiling of 16); an explicit -x is honored
-// verbatim, including the aria2-faithful -x1 single connection.
+// verbatim, including -x1 for a single connection.
 func TestHTTPConnDefault(t *testing.T) {
 	cfg := func(args ...string) httpdl.Config {
 		res, err := cli.Parse(append([]string{"--no-conf"}, args...))