Patch #1714700: clarify os.linesep vs. tfiles opened in text mode.
(backport)
diff --git a/Doc/lib/libos.tex b/Doc/lib/libos.tex
index 7844028..18c5122 100644
--- a/Doc/lib/libos.tex
+++ b/Doc/lib/libos.tex
@@ -1973,9 +1973,12 @@
\begin{datadesc}{linesep}
The string used to separate (or, rather, terminate) lines on the
-current platform. This may be a single character, such as \code{'\e
-n'} for \POSIX{} or \code{'\e r'} for Mac OS, or multiple characters,
-for example, \code{'\e r\e n'} for Windows.
+current platform. This may be a single character, such as
+\code{'\e n'} for \POSIX{} or \code{'\e r'} for Mac OS, or multiple
+characters, for example, \code{'\e r\e n'} for Windows.
+Do not use \var{os.linesep} as a line terminator when writing files
+opened in text mode (the default); use a single \code{'\e n'} instead,
+on all platforms.
\end{datadesc}
\begin{datadesc}{devnull}