Fix #328: Add note about maxsize being an actual number.

Author: tkem

docs/index.rst

@@ -41,6 +41,12 @@ of the cache.  When a cache is full, :meth:`Cache.__setitem__()` calls
 :meth:`self.popitem()` repeatedly until there is enough room for the
 item to be added.
 
+.. note::
+
+   Please be aware that `maxsize` must be a positive number.  If you
+   really want your cache to grow without bounds, use
+   :const:`math.inf` or something similar.
+
 In general, a cache's size is the total size of its item's values.
 Therefore, :class:`Cache` provides a :meth:`getsizeof` method, which
 returns the size of a given `value`.  The default implementation of
@@ -50,12 +56,12 @@ making the cache's size equal to the number of its items, or
 named constructor parameter `getsizeof`, which may specify a function
 of one argument used to retrieve the size of an item's value.
 
-Note that the values of a :class:`Cache` are mutable by default, as
-are e.g. the values of a :class:`dict`.  It is the user's
-responsibility to take care that cached values are not accidentally
-modified.  This is especially important when using a custom
-`getsizeof` function, since the size of an item's value will only be
-computed when the item is inserted into the cache.
+The values of a :class:`Cache` are mutable by default, as are e.g. the
+values of a :class:`dict`.  It is the user's responsibility to take
+care that cached values are not accidentally modified.  This is
+especially important when using a custom `getsizeof` function, since
+the size of an item's value will only be computed when the item is
+inserted into the cache.
 
 .. note::