(Narrowing): Add an xref to "Swapping Text".

doc/lispref/positions.texi

@@ -912,6 +912,10 @@ which use them refuse to operate on text that is inaccessible.
   The commands for saving buffers are unaffected by narrowing; they save
 the entire buffer regardless of any narrowing.
 
+  If you need to display in a single buffer several very different
+types of text, consider using an alternative facility described in
+@ref{Swapping Text}.
+
 @deffn Command narrow-to-region start end
 This function sets the accessible portion of the current buffer to start
 at @var{start} and end at @var{end}.  Both arguments should be character