bpo-43126: Expand docs on io.IOBase.readlines() method (GH-27061) (#27114)

Co-authored-by: Łukasz Langa <[email protected]>
(cherry picked from commit 3b5b99d)

Co-authored-by: andrei kulakov <[email protected]>

Author: miss-islington

Doc/library/io.rst

@@ -315,6 +315,9 @@ 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()``.