[pprint]: Add docstring about `PrettyPrinter.underscore_numbers` parameter (#112963)

Co-authored-by: Kirill Podoprigora <kirill.bast9@mail.ru>

Lib/pprint.py

@@ -128,6 +128,9 @@ class PrettyPrinter:
         sort_dicts
             If true, dict keys are sorted.
 
+        underscore_numbers
+            If true, digit groups are separated with underscores.
+
         """
         indent = int(indent)
         width = int(width)