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.go

@@ -509,7 +509,7 @@ func httpConfig(opts *cli.Options, single bool, overallDL *rate.Limiter) httpdl.
 	if single {
 		out = opts.Str("out")
 	}
-	// max-connection-per-server caps connections per host. aria2's default is 1,
+	// max-connection-per-server caps connections per host. Its default is 1,
 	// which on its own would clamp --split to a single connection, so a bare
 	// `got URL` would download single-threaded. Honor the literal default of 1
 	// only when the user actually set -x; otherwise let --split drive per-host