Document legacy exceptions in their own section.

This change has the side effect of dropping the WebSocketProtocolError
exception from the list. It was documented as an alias of ProtocolError
as a side effect of using the automodule directive. Eventually it will
get deprecated with the rest of the legacy module so potential users
will be aware.

Author: aaugustin

docs/reference/exceptions.rst

@@ -2,5 +2,56 @@ Exceptions
 ==========
 
 .. automodule:: websockets.exceptions
-    :members:
 
+.. autoexception:: WebSocketException
+
+.. autoexception:: ConnectionClosed
+
+.. autoexception:: ConnectionClosedError
+
+.. autoexception:: ConnectionClosedOK
+
+.. autoexception:: InvalidHandshake
+
+.. autoexception:: SecurityError
+
+.. autoexception:: InvalidHeader
+
+.. autoexception:: InvalidHeaderFormat
+
+.. autoexception:: InvalidHeaderValue
+
+.. autoexception:: InvalidOrigin
+
+.. autoexception:: InvalidUpgrade
+
+.. autoexception:: InvalidStatus
+
+.. autoexception:: NegotiationError
+
+.. autoexception:: DuplicateParameter
+
+.. autoexception:: InvalidParameterName
+
+.. autoexception:: InvalidParameterValue
+
+.. autoexception:: InvalidState
+
+.. autoexception:: InvalidURI
+
+.. autoexception:: PayloadTooBig
+
+.. autoexception:: ProtocolError
+
+Legacy exceptions
+-----------------
+
+These exceptions are only used by the legacy :mod:`asyncio` implementation.
+
+.. autoexception:: InvalidMessage
+
+.. autoexception:: InvalidStatusCode
+
+.. autoexception:: AbortHandshake
+
+.. autoexception:: RedirectHandshake