chore: update PhonenumberSerializer docstring

openedx/core/djangoapps/user_api/accounts/serializers.py

@@ -48,11 +48,23 @@
 
 class PhoneNumberSerializer(serializers.BaseSerializer):  # lint-amnesty, pylint: disable=abstract-method
     """
-    Class to serialize phone number into a digit only representation
+    Class to serialize phone number into a digit only representation.
+
+    This serializer removes all non-numeric characters from the phone number,
+    allowing '+' only at the beginning of the number.
     """
 
     def to_internal_value(self, data):
-        """Remove all non numeric characters in phone number"""
+        """
+        Remove all non-numeric characters from the phone number.
+
+        Args:
+            data (str): The input phone number string.
+
+        Returns:
+            str or None: The cleaned phone number string containing only digits,
+                with an optional '+' at the beginning.
+        """
         return re.sub(r'(?!^)\+|[^0-9+]', "", data) or None