Document 'istext' parameter
diff --git a/Doc/lib/libbinascii.tex b/Doc/lib/libbinascii.tex
index 9850418..b244b10 100644
--- a/Doc/lib/libbinascii.tex
+++ b/Doc/lib/libbinascii.tex
@@ -51,10 +51,13 @@
Convert binary data to a line(s) of \ASCII{} characters in
quoted-printable encoding. The return value is the converted line(s).
If the optional argument \var{quotetabs} is present and true, all tabs
-and spaces will be encoded. If the optional argument \var{header} is
+and spaces will be encoded.
+If the optional argument \var{istext} is present and true,
+newlines are not encoded but trailing whitespace will be encoded.
+If the optional argument \var{header} is
present and true, spaces will be encoded as underscores per RFC1522.
If the optional argument \var{header} is present and false, newline
-characters will be encoded as well, otherwise linefeed conversion might
+characters will be encoded as well; otherwise linefeed conversion might
corrupt the binary data stream.
\end{funcdesc}