Restructure tz docs to be more organized

docs/tz.rst

 ==
 tz
 ==
-.. autofunction:: dateutil.tz.gettz
+.. py:currentmodule:: dateutil.tz
 
 .. automodule:: dateutil.tz
@@ -5,3 +5,40 @@
 
 .. automodule:: dateutil.tz
+
+Objects
+-------
+.. py:data:: dateutil.tz.UTC
+
+    A convenience instance of :class:`dateutil.tz.tzutc`.
+
+Functions
+---------
+
+.. autofunction:: gettz
+
+    .. automethod:: gettz.nocache
+    .. automethod:: gettz.cache_clear
+
+.. autofunction:: enfold
+
+.. autofunction:: datetime_ambiguous
+.. autofunction:: datetime_exists
+
+.. autofunction:: resolve_imaginary
+
+
+Classes
+-------
+
+.. autoclass:: tzutc
+
+.. autoclass:: tzoffset
+
+.. autoclass:: tzlocal
+
+.. autoclass:: tzrange
+
+.. autoclass:: tzstr
+
+.. autoclass:: tzical
     :members:
@@ -7,2 +44,1 @@
     :members:
-   :undoc-members: