Clarify that quit() and close() render the instance useless.
diff --git a/Doc/lib/libftplib.tex b/Doc/lib/libftplib.tex
index d2358e8..166fd6e 100644
--- a/Doc/lib/libftplib.tex
+++ b/Doc/lib/libftplib.tex
@@ -234,11 +234,16 @@
Send a \samp{QUIT} command to the server and close the connection.
This is the ``polite'' way to close a connection, but it may raise an
exception of the server reponds with an error to the \samp{QUIT}
-command.
+command. This implies a call to the \method{close()} method which
+renders the \class{FTP} instance useless for subsequent calls (see
+below).
\end{methoddesc}
\begin{methoddesc}{close}{}
Close the connection unilaterally. This should not be applied to an
already closed connection (e.g.\ after a successful call to
-\method{quit()}.
+\method{quit()}. After this call the \class{FTP} instance should not
+be used any more (i.e., after a call to \method{close()} or
+\method{quit()} you cannot reopen the connection by issueing another
+\method{login()} method).
\end{methoddesc}