Fixes gh-682 Document new behaviour of session.on_disconnect

Author: pgulutzan

doc/1.10/book/box/box_session.rst

@@ -79,7 +79,7 @@ Below is a list of all ``box.session`` functions and members.
 .. function:: id()
 
     :return: the unique identifier (ID) for the current session.
-             The result can be 0 meaning there is no session.
+             The result can be 0 or -1 meaning there is no session.
     :rtype:  number
 
 .. _box_session-exists:
@@ -114,6 +114,7 @@ Below is a list of all ``box.session`` functions and members.
 
     :return: the value of the :code:`sync` integer constant used in the
              `binary protocol <https://github.com/tarantool/tarantool/blob/1.10/src/box/iproto_constants.h>`_.
+             This value becomes invalid when the session is disconnected.
 
     :rtype:  number
 
@@ -330,6 +331,9 @@ Below is a list of all ``box.session`` functions and members.
     trigger is invoked while the session associated with the client still exists
     and can access session properties, such as :ref:`box.session.id() <box_session-id>`.
 
+    Since version 1.10, the trigger function is invoked immediately after the disconnect,
+    even if requests that were made during the session have not finished.
+
     :param function trigger-function: function which will become the trigger function
     :param function old-trigger-function: existing trigger function which will be replaced by trigger-function
     :return: nil or function pointer
@@ -495,6 +499,8 @@ Below is a list of all ``box.session`` functions and members.
     our terminology for net.box), and the client has options for getting
     such messages.
 
+    This function returns an error if the session is disconnected.
+
     :param string-or-number message: what to send
     :param int sync: an optional argument to indicate what the session is,
                      as taken from an earlier call to :ref:`box_session:sync() <box_session-sync>`.