Added a note about mutable types of named tuples

As pointed out by Cornelius Krupp, types created
by the namedtuple constructor are mutable.

Author: TobiasWrigstad

peps/pep-0795.rst

@@ -268,6 +268,11 @@ across sub-interpreters without the changes proposed by this PEP. The
 ``n`` tuple contains a mutable dictionary and is, therefore, **not**
 deeply immutable and therefore not safe to share between sub-interpreters.
 
+(Note that types created by ``namedtuple`` are mutable. This means that
+``Person`` above is mutable. Below we introduce a solution for this by
+way of a ``freeze()`` function. Another possibility is to provide a
+companion constructor for named tuples that create immutable types.)
+
 
 Sharing of Types
 ~~~~~~~~~~~~~~~~