pool: add __doc__

gevent/pool.py

 # Copyright (c) 2009-2010 Denis Bilenko. See LICENSE for details.
+"""Utilities for managing greenlets in a group.
+
+The :class:`Group` class in this module abstracts a group of running greenlets.
+When a greenlet dies, it's automatically removed from the group.
+
+The :class:`Pool` which a subclass of :class:`Group` provides a way to limit
+concurrency: its :meth:`spawn <Pool.spawn>` blocks if the number of greenlets
+in the pool would exceed the limit.
+"""
 
 from gevent.hub import GreenletExit, getcurrent
 from gevent.greenlet import joinall, Greenlet