Clarifiy and update usage.rst

relevant to #230
clarifies the config-file  in the usage.rst

doc/user/usage.rst

@@ -1,6 +1,7 @@
 *****
 Usage
 *****
+:Last Reviewed: 2017-06-10
 
 Define your printer
 -------------------
@@ -133,13 +134,13 @@ format. For windows it is probably at::
 
 And for linux::
 
-        $HOME/.config/python-escpos/config.yaml
+    $HOME/.config/python-escpos/config.yaml
 
 If you aren't sure, run::
 
-        from escpos import config
-        c = config.Config()
-        c.load()
+    from escpos import config
+    c = config.Config()
+    c.load()
 
 If it can't find the configuration file in the default location, it will tell
 you where it's looking. You can always pass a path, or a list of paths, to
@@ -147,9 +148,9 @@ the ``load()`` method.
 
 To load the configured printer, run::
 
-        from escpos import config
-        c = config.Config()
-        printer = c.printer()
+    from escpos import config
+    c = config.Config()
+    printer = c.printer()
 
 
 The printer section
@@ -157,23 +158,34 @@ The printer section
 
 The ``printer`` configuration section defines a default printer to create.
 
-The only required paramter is ``type``. The value of this should be one of the
+The only required paramter is ``type``. The value of this has to be one of the
 printers defined in :doc:`/user/printers`.
 
-The rest of the parameters are whatever you want to pass to the printer.
+The rest of the given parameters will be passed on to the initialization of the printer class.
+Use these to overwrite the default values as specified in :doc:`/user/printers`.
+This implies that the parameters have to match the parameter-names of the respective printer class.
 
 An example file printer::
 
-        printer:
-                type: File
-                devfile: /dev/someprinter
+    printer:
+            type: File
+            devfile: /dev/someprinter
 
 And for a network printer::
 
-        printer:
-                type: network
-                host: 127.0.0.1
-                port: 9000
+    printer:
+            type: Network
+            host: 127.0.0.1
+            port: 9000
+
+An USB-printer could be defined by::
+
+    printer:
+            type: Usb
+            idVendor: 0x1234
+            idProduct: 0x5678
+            in_ep: 0x66
+            out_ep: 0x01
 
 Printing text right
 -------------------