Document "multifile" as deprecated.

Doc/lib/libmultifile.tex

@@ -6,6 +6,10 @@
                 parts, such as some MIME data.}
 \sectionauthor{Eric S. Raymond}{esr@snark.thyrsus.com}
 
+\deprecated{2.5}{The \refmodule{email} package should be used in
+                 preference to the \module{multifile} module.
+                 This module is present only to maintain backward
+                 compatibility.}
 
 The \class{MultiFile} object enables you to treat sections of a text
 file as file-like input objects, with \code{''} being returned by