Document the HttpClient uses KeepAlive, keeping sockets open for up to 60 seconds after a connectio…

runtime/bin/http.dart

Index: runtime/bin/http.dart
diff --git a/runtime/bin/http.dart b/runtime/bin/http.dart
index d18ed339d2eb0ded2678f0d3d9e56cd026d5922a..be5d21f5f3d4bb76b4320ef202eebe95ec7606cd 100644
--- a/runtime/bin/http.dart
+++ b/runtime/bin/http.dart
@@ -623,7 +623,13 @@ interface HttpResponse default _HttpResponse {
 
 
 /**
- * HTTP client factory.
+ * HTTP client factory. The [HttpClient] handles all the sockets associated
+ * with the [HttpClientConnection]s and when the endpoint supports it, it will
+ * try to reuse opened sockets for several requests to support HTTP 1.1
+ * persistent connections. This means that sockets will be kept open for some
+ * time after a requests have completed, unless HTTP procedures indicate that it
+ * must be closed as part of completing the request. Use [:HttpClient.shutdown:]
+ * to force close the idle sockets.
  */
 interface HttpClient default _HttpClient {
   static final int DEFAULT_HTTP_PORT = 80;