bpo-43126: Expand docs on io.IOBase.readlines() method (python#27061)

Co-authored-by: Łukasz Langa <lukasz@langa.pl>

Author: akulakov

Doc/library/io.rst

@@ -391,6 +391,9 @@ I/O Base Classes
       to control the number of lines read: no more lines will be read if the
       total size (in bytes/characters) of all lines so far exceeds *hint*.
 
+      *hint* values of ``0`` or less, as well as ``None``, are treated as no
+      hint.
+
       Note that it's already possible to iterate on file objects using ``for
       line in file: ...`` without calling ``file.readlines()``.