Explain what os.read() returns at end of file.
This closes SF bug #550409. Applying to release21-maint & release22-maint.
diff --git a/Doc/lib/libos.tex b/Doc/lib/libos.tex
index 834e562..c956ab1 100644
--- a/Doc/lib/libos.tex
+++ b/Doc/lib/libos.tex
@@ -471,7 +471,9 @@
\begin{funcdesc}{read}{fd, n}
Read at most \var{n} bytes from file descriptor \var{fd}.
-Return a string containing the bytes read.
+Return a string containing the bytes read. If the end of the file
+referred to by \var{fd} has been reached, an empty string is
+returned.
Availability: Macintosh, \UNIX, Windows.
Note: this function is intended for low-level I/O and must be applied