[concept.swappable] Add note explaining why swap poison pill is still needed

Author: danra

source/concepts.tex

@@ -620,6 +620,11 @@
   exchange the values denoted by
   \tcode{E1} and \tcode{E2},
   the program is ill-formed, no diagnostic required.
+  \begin{note}
+  This precludes calling unconstrained program-defined overloads of
+  \tcode{swap}. When the deleted overload is viable, program-defined overloads
+  need to be more specialized\iref{temp.func.order} to be selected.
+  \end{note}
 
 \item
   Otherwise, if \tcode{E1} and \tcode{E2}