diff --git a/Doc/library/io.rst b/Doc/library/io.rst
index 96e02e839ae..0ff05cfc0e0 100644
--- a/Doc/library/io.rst
+++ b/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()``.