docs: add explanation about a shared instance and parameters

Author: kenjis

user_guide_src/source/concepts/services.rst

@@ -36,7 +36,7 @@ come in handy.
 
 Instead of creating the instance ourself, we let a central class create an instance of the
 class for us. This class is kept very simple. It only contains a method for each class that we want
-to use as a service. The method typically returns a shared instance of that class, passing any dependencies
+to use as a service. The method typically returns a **shared instance** of that class, passing any dependencies
 it might have into it. Then, we would replace our timer creation code with code that calls this new class:
 
 .. literalinclude:: services/002.php
@@ -57,6 +57,15 @@ As many CodeIgniter classes are provided as services, you can get them like the
 
 The ``$typography`` is an instance of the Typography class, and if you call ``\Config\Services::typography()`` again, you will get the exactly same instance.
 
+The Services typically return a **shared instance** of the class. The following code creates a ``CURLRequest`` instance at the first call. And the second call returns the exactly same instance.
+
+.. literalinclude:: services/015.php
+
+Therefore, the parameter ``$options2`` for the ``$client2`` does not work. It is just ignored.
+
+Getting a New Instance
+======================
+
 If you want to get a new instance of the Typography class, you need to pass ``false`` to the argument ``$getShared``:
 
 .. literalinclude:: services/014.php

user_guide_src/source/concepts/services/015.php

@@ -0,0 +1,15 @@
+<?php
+
+$options1 = [
+    'baseURI' => 'http://example.com/api/v1/',
+    'timeout' => 3,
+];
+$client1 = \Config\Services::curlrequest($options1);
+
+$options2 = [
+    'baseURI' => 'http://another.example.com/api/v2/',
+    'timeout' => 10,
+];
+$client2 = \Config\Services::curlrequest($options2);
+// $options2 does not work.
+// $client2 is the exactly same instance as $client1.