Add target node documentation

Nothing in the docs currently shows how pylxd can specify the target node on creation. The location can be read from the instance and is listed as an attribute, but specifying 'location' in the config on creation does not create the desired result.

doc/source/instances.rst

@@ -20,11 +20,11 @@ methods:
   - `exists(name)` - Returns `boolean` indicating if the instance exists.
   - `all()` - Retrieve all instances.
   - `get()` - Get a specific instance, by its name.
-  - `create(config, wait=False)` - Create a new instance. This method
-    requires the instance config as the first parameter.
-    The config itself is beyond the scope of this documentation. Please
-    refer to the LXD documentation for more information. This method
-    will also return immediately, unless `wait` is `True`.
+  - `create(config, wait=False, target='lxd-cluster-member')` - Create a new instance. 
+    - This method requires the instance config as the first parameter.
+    - The config itself is beyond the scope of this documentation. Please refer to the LXD documentation for more information. 
+    - This method will also return immediately, unless `wait` is `True`.
+    - Optionally, the target node can be specified for LXD clusters.
 
 
 Instance attributes