ddtrace/tracer: add link to Service page and Inferred Services (#2542)

ddtrace/tracer/option.go

@@ -874,8 +874,8 @@ func WithServiceMapping(from, to string) StartOption {
 }
 
 // WithPeerServiceDefaults sets default calculation for peer.service.
+// Related documentation: https://docs.datadoghq.com/tracing/guide/inferred-service-opt-in/?tab=go#apm-tracer-configuration
 func WithPeerServiceDefaults(enabled bool) StartOption {
-	// TODO: add link to public docs
 	return func(c *config) {
 		c.peerServiceDefaultsEnabled = enabled
 	}