Document 'istext' parameter

Doc/lib/libbinascii.tex

@@ -51,10 +51,13 @@ will be decoded as spaces.
 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}