Improve NewRR documentation (#1531)

In particular, document the default origin.

scan.go

@@ -101,12 +101,13 @@ type ttlState struct {
 	isByDirective bool   // isByDirective indicates whether ttl was set by a $TTL directive
 }
 
-// NewRR reads the RR contained in the string s. Only the first RR is returned.
+// NewRR reads a string s and returns the first RR.
 // If s contains no records, NewRR will return nil with no error.
 //
-// The class defaults to IN and TTL defaults to 3600. The full zone file syntax
-// like $TTL, $ORIGIN, etc. is supported. All fields of the returned RR are
-// set, except RR.Header().Rdlength which is set to 0.
+// The class defaults to IN, TTL defaults to 3600, and
+// origin for resolving relative domain names defaults to the DNS root (.).
+// Full zone file syntax is supported, including directives like $TTL and $ORIGIN.
+// All fields of the returned RR are set from the read data, except RR.Header().Rdlength which is set to 0.
 func NewRR(s string) (RR, error) {
 	if len(s) > 0 && s[len(s)-1] != '\n' { // We need a closing newline
 		return ReadRR(strings.NewReader(s+"\n"), "")