Documented the new error_callback keyword argument to multiprocessing.Pool's…

Documented the new error_callback keyword argument to multiprocessing.Pool's apply_async and map_async

Doc/library/multiprocessing.rst

@@ -1582,14 +1582,21 @@ with the :class:`Pool` class.
       suited for performing work in parallel. Additionally, the passed in
       function is only executed in one of the workers of the pool.
 
-   .. method:: apply_async(func[, args[, kwds[, callback]]])
+   .. method:: apply_async(func[, args[, kwds[, callback[, error_callback]]]])
 
       A variant of the :meth:`apply` method which returns a result object.
 
       If *callback* is specified then it should be a callable which accepts a
       single argument.  When the result becomes ready *callback* is applied to
-      it (unless the call failed).  *callback* should complete immediately since
-      otherwise the thread which handles the results will get blocked.
+      it, that is unless the call failed, in which case the *error_callback*
+      is applied instead
+
+      If *error_callback* is specified then it should be a callable which
+      accepts a single argument.  If the target function fails, then
+      the *error_callback* is called with the exception instance.
+
+      Callbacks should complete immediately since otherwise the thread which
+      handles the results will get blocked.
 
    .. method:: map(func, iterable[, chunksize])
 
@@ -1606,8 +1613,15 @@ with the :class:`Pool` class.
 
       If *callback* is specified then it should be a callable which accepts a
       single argument.  When the result becomes ready *callback* is applied to
-      it (unless the call failed).  *callback* should complete immediately since
-      otherwise the thread which handles the results will get blocked.
+      it, that is unless the call failed, in which case the *error_callback*
+      is applied instead
+
+      If *error_callback* is specified then it should be a callable which
+      accepts a single argument.  If the target function fails, then
+      the *error_callback* is called with the exception instance.
+
+      Callbacks should complete immediately since otherwise the thread which
+      handles the results will get blocked.
 
    .. method:: imap(func, iterable[, chunksize])