Update ag2_exact_traits_sqrt.cpp

[Apollonius_graph_2][examples] add warnings (in the form of comments)
indicating that an exact kernel should not be chosen when efficiency is an issue.

Apollonius_graph_2/examples/Apollonius_graph_2/ag2_exact_traits_sqrt.cpp

@@ -12,6 +12,12 @@
 #  include <CGAL/CORE_Expr.h>
 #endif
 
+// *** WARNING ***
+// The use of a kernel based on an exact number type is highly inefficient.
+// It is used in this example primarily for illustration purposes.
+// In an efficiency critical context, and/or for the purposes of
+// benchmarking the Apollonius_graph_filtered_traits_2<> class should
+// be used.
 
 #if defined CGAL_USE_LEDA
 // If LEDA is present use leda_real as the exact number type