gh-150403: Document frozendict in language reference Mappings section

Author: oedokumaci

Doc/reference/datamodel.rst

@@ -496,7 +496,7 @@ subscript notation ``a[k]`` selects the item indexed by ``k`` from the mapping
 :keyword:`del` statements. The built-in function :func:`len` returns the number
 of items in a mapping.
 
-There is currently a single intrinsic mapping type:
+There are currently two intrinsic mapping types:
 
 
 Dictionaries
@@ -535,6 +535,20 @@ module.
    an implementation detail at that time rather than a language guarantee.
 
 
+Frozen dictionaries
+^^^^^^^^^^^^^^^^^^^
+
+.. index:: pair: object; frozendict
+
+These represent an immutable dictionary.  They are created by the built-in
+:func:`frozendict` constructor.  A frozendict is :term:`hashable` if all of
+its keys and values are hashable, in which case it can be used as an element
+of a set, or as a key in another mapping.  :class:`!frozendict` is not a
+subclass of :class:`dict`; it inherits directly from :class:`object`.
+
+.. versionadded:: 3.15
+
+
 Callable types
 --------------