Issue 15265: document the exception raised for invalid sample sizes.

Doc/library/random.rst

@@ -150,6 +150,9 @@ Functions for sequences:
    argument.  This is especially fast and space efficient for sampling from a large
    population:  ``sample(range(10000000), 60)``.
 
+   If the sample size is larger than the population size, a :exc:``ValueError``
+   is raised.
+
 The following functions generate specific real-valued distributions. Function
 parameters are named after the corresponding variables in the distribution's
 equation, as used in common mathematical practice; most of these equations can