Add a note about lightweight mode in the man

doc/mat2.1

@@ -71,6 +71,13 @@ complex file formats.
 .PP
 This is why you shouldn't rely on metadata's presence to decide if your file must
 be cleaned or not.
+.PP
+Moreover, mat2 goes to great lengths to make sure that as much metadata as
+possible are removed. This might sometimes results in a loss of quality of the
+processed files. If you're experiencing this, you might want to give a try at
+the lightweigtht cleaning mode, but keep in mind by doing so, some metadata
+\fBwon't be cleaned\fR.
+
 
 .SH BUGS