Added documentation for is_open and isOpen.

I'm not sure if isOpen() has been officially deprecated, so I just marked it as such.

Author: Matthew West

documentation/pyserial_api.rst

@@ -275,6 +275,9 @@ Native ports
 
         Return the state of the CD line
 
+    .. attribute:: is_open
+    	:getter: Get the state of the serial port, whether it's open.
+    	:type: bool
 
     New values can be assigned to the following attributes (properties), the
     port will be reconfigured, even if it's opened at that time:
@@ -564,6 +567,10 @@ Native ports
 
         .. deprecated:: 3.0 see :attr:`in_waiting`
 
+    .. method:: isOpen()
+
+    	.. deprecated:: 3.0 see :attr:`is_open`
+
     .. attribute:: writeTimeout
 
         .. deprecated:: 3.0 see :attr:`write_timeout`