Add usage notes for collections.Counter()
diff --git a/Doc/library/collections.rst b/Doc/library/collections.rst
index e5036d5..d084179 100644
--- a/Doc/library/collections.rst
+++ b/Doc/library/collections.rst
@@ -271,6 +271,33 @@
>>> c | d # union: max(c[x], d[x])
Counter({'a': 3, 'b': 2})
+.. note::
+
+ Counters were primarily designed to work with positive integers to represent
+ running counts; however, care was taken to not unnecessarily preclude use
+ cases needing other types or negative values. To help with those use cases,
+ this section documents the minimum range and type restrictions.
+
+ * The :class:`Counter` class itself is a dictionary subclass with no
+ restrictions on its keys and values. The values are intended to be numbers
+ representing counts, but you *could* store anything in the value field.
+
+ * The :meth:`most_common` method requires only that the values be orderable.
+
+ * For in-place operations such as ``c[key] += 1``, the value type need only
+ support addition and subtraction. So fractions, floats, and decimals would
+ work and negative values are supported. The same is also true for
+ :meth:`update` and :meth:`subtract` which allow negative and zero values
+ for both inputs and outputs.
+
+ * The multiset methods are designed only for use cases with positive values.
+ The inputs may be negative or zero, but only outputs with positive values
+ are created. There are no type restrictions, but the value type needs to
+ support support addition, subtraction, and comparison.
+
+ * The :meth:`elements` method requires integer counts. It ignores zero and
+ negative counts.
+
.. seealso::
* `Counter class <http://code.activestate.com/recipes/576611/>`_