Issue #21171: Fixed undocumented filter API of the rot13 codec.

Patch by Berker Peksag.

diff --cc Misc/NEWS
index 0b558727ba1cbdfe55f4ea89c73a70e934920247,f381c93109b01ca46432c10664b9de7fced5d1d5..949d8e69772ef6a85b0aac5dda63bed44999087a
--- 1/Misc/NEWS
--- 2/Misc/NEWS
+++ b/Misc/NEWS
@@@ -34,10 -27,9 +34,13 @@@ Core and Builtin
  Library
  -------
  
+ - Issue #21171: Fixed undocumented filter API of the rot13 codec.
+   Patch by Berker Peksag.
+ 
 +- Issue #20539: Improved math.factorial error message for large positive inputs
 +  and changed exception type (OverflowError -> ValueError) for large negative
 +  inputs.
 +
  - Issue #21172: isinstance check relaxed from dict to collections.Mapping.
  
  - Issue #21155: asyncio.EventLoop.create_unix_server() now raises a ValueError