docs/library/pyb.USB_VCP.rst: Add info about id and flow params.

docs/library/pyb.USB_VCP.rst

@@ -12,14 +12,21 @@ the connected host.
 Constructors
 ------------
 
-.. class:: pyb.USB_VCP()
+.. class:: pyb.USB_VCP(id=0)
 
-   Create a new USB_VCP object.
+   Create a new USB_VCP object.  The *id* argument specifies which USB VCP port to
+   use.
 
 
 Methods
 -------
 
+.. method:: USB_VCP.init(\*, flow=-1)
+
+   Configure the USB VCP port.  If the *flow* argument is not -1 then the value sets
+   the flow control, which can be a bitwise-or of ``USB_VCP.RTS`` and ``USB_VCP.CTS``.
+   RTS is used to control read behaviour and CTS, to control write behaviour.
+
 .. method:: USB_VCP.setinterrupt(chr)
 
    Set the character which interrupts running Python code.  This is set
@@ -101,3 +108,12 @@ Methods
      - ``timeout`` is the timeout in milliseconds to wait for the send.
    
    Return value: number of bytes sent.
+
+
+Constants
+---------
+
+.. data:: USB_VCP.RTS
+          USB_VCP.CTS
+
+   to select the flow control type.