Clarify the effect of text mode.
diff --git a/Doc/library/functions.rst b/Doc/library/functions.rst
index 622ff5f4..665b865 100644
--- a/Doc/library/functions.rst
+++ b/Doc/library/functions.rst
@@ -734,7 +734,9 @@
writing (truncating the file if it already exists), and ``'a'`` for appending
(which on *some* Unix systems means that *all* writes append to the end of the
file regardless of the current seek position). If *mode* is omitted, it
- defaults to ``'r'``. When opening a binary file, you should append ``'b'`` to
+ defaults to ``'r'``. The default is to use text mode, which may convert
+ ``'\n'`` characters to a platform-specific representation on writing and back
+ on reading. Thus, when opening a binary file, you should append ``'b'`` to
the *mode* value to open the file in binary mode, which will improve
portability. (Appending ``'b'`` is useful even on systems that don't treat
binary and text files differently, where it serves as documentation.) See below