Add raised exceptions to authenticate method doc

Closes #111

Author: rogeriopvl

lib/pusher/channel.rb

@@ -120,6 +120,8 @@ def users(params = {})
     # @param custom_string [String] Allows signing additional data
     # @return [String]
     #
+    # @raise [Pusher::Error] if socket_id or custom_string invalid
+    #
     def authentication_string(socket_id, custom_string = nil)
       validate_socket_id(socket_id)
 
@@ -157,6 +159,8 @@ def authentication_string(socket_id, custom_string = nil)
     #
     # @return [Hash]
     #
+    # @raise [Pusher::Error] if socket_id or custom_data is invalid
+    #
     # @private Custom data is sent to server as JSON-encoded string
     #
     def authenticate(socket_id, custom_data = nil)

lib/pusher/client.rb

@@ -345,6 +345,8 @@ def notify(interests, data = {})
     #
     # @return [Hash]
     #
+    # @raise [Pusher::Error] if channel_name or socket_id are invalid
+    #
     # @private Custom data is sent to server as JSON-encoded string
     #
     def authenticate(channel_name, socket_id, custom_data = nil)