Add note about missing itertools functions (#117)

Author: ziima

docs/source/api/itertools.rst

@@ -8,6 +8,12 @@ The itertools library
 The :py:mod:`asyncstdlib.itertools` library implements
 Python's :py:mod:`itertools` for (async) functions and (async) iterables.
 
+.. note::
+
+    Only functions which benefit from an explicit ``async`` implementation are provided.
+    Other functions from ``itertools`` can be turned asynchronous using :py:func:`~asyncstdlib.iter`,
+    e.g. `asyncstdlib.iter(itertools.count(5))`.
+
 .. note::
 
     To avoid leaking resources, all utilities in this module explicitly close their