[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 @@ def __init__(self, indent=1, width=80, depth=None, stream=None, *,
         sort_dicts
             If true, dict keys are sorted.
 
+        underscore_numbers
+            If true, digit groups are separated with underscores.
+
         """
         indent = int(indent)
         width = int(width)