Added a note describing how fp should be opened in binary mode, especially on

Windows.  Closes SF # 586899.

Doc/lib/libmailbox.tex

@@ -18,6 +18,10 @@ argument, \var{fp} by the \method{next()} method of the mailbox
 object.  The default is the \class{rfc822.Message} class (see the
 \refmodule{rfc822} module -- and the note below).
 
+\note{For reasons of this module's internal implementation, you will probably
+want to open the \var{fp} object in binary mode.  This is especially important
+on Windows.}
+
 For maximum portability, messages in a \UNIX-style mailbox are
 separated by any line that begins exactly with the string \code{'From
 '} (note the trailing space) if preceded by exactly two newlines.