clarify warning message to avoid confusing beginners

--- a/src/Provers/Arith/fast_lin_arith.ML	Mon Jun 27 13:52:47 2011 +0200
+++ b/src/Provers/Arith/fast_lin_arith.ML	Mon Jun 27 13:52:47 2011 +0200
@@ -633,7 +633,7 @@
           handle NoEx => NONE
       in
         case ex of
-          SOME s => (warning "Linear arithmetic failed - see trace for a counterexample."; tracing s)
+          SOME s => (warning "Linear arithmetic failed - see trace for a (potentially spurious) counterexample."; tracing s)
         | NONE => warning "Linear arithmetic failed"
       end;