docs: Add IO context manager description

Signed-off-by: Sven Sager <akira@narux.de>

src/revpimodio2/io.py

@@ -88,6 +88,17 @@ class IOList(object):
         io_del._parentdevice._update_my_io_list()
 
     def __enter__(self):
+        """
+        Read inputs on entering context manager and write outputs on leaving.
+
+        All entries are read when entering the context manager. Within the
+        context manager, further .readprocimg() or .writeprocimg() calls can
+        be made and the process image can be read or written. When exiting,
+        all outputs are always written into the process image.
+
+        When 'autorefresh=True' is used, all read or write actions in the
+        background are performed automatically.
+        """
         if self.__modio._looprunning:
             raise RuntimeError("can not start multiple mainloop/cycleloop/with sessions")
         self.__modio._looprunning = True
@@ -96,6 +107,12 @@ class IOList(object):
         return self
 
     def __exit__(self, exc_type, exc_val, exc_tb):
+        """
+        Write outputs to process image before leaving the context manager.
+
+        If 'autorefresh=True' is used, this operation is asynchronous. The
+        outputs are automatically written in the background.
+        """
         self.__modio.writeprocimg()
         self.__modio._looprunning = False