Fix the wording of the Scalar::Util::export_fail diagnostic messages;…

… also document the isbool failure one

cpan/Scalar-List-Utils/lib/Scalar/Util.pm

@@ -40,17 +40,17 @@ unless (defined &isvstring) {
 sub export_fail {
   if (grep { /^(?:weaken|isweak)$/ } @_ ) {
     require Carp;
-    Carp::croak("Weak references are not implemented in the version of perl");
+    Carp::croak("Weak references are not implemented in this version of perl");
   }
 
   if (grep { /^isvstring$/ } @_ ) {
     require Carp;
-    Carp::croak("Vstrings are not implemented in the version of perl");
+    Carp::croak("Vstrings are not implemented in this version of perl");
   }
 
   if (grep { /^isbool$/ } @_ ) {
     require Carp;
-    Carp::croak("isbool is not implemented in the version of perl");
+    Carp::croak("isbool is not implemented in this version of perl");
   }
 
   @_;
@@ -341,16 +341,21 @@ Module use may give one of the following errors during import.
 
 =over
 
-=item Weak references are not implemented in the version of perl
+=item Weak references are not implemented in this version of perl
 
 The version of perl that you are using does not implement weak references, to
 use L</isweak> or L</weaken> you will need to use a newer release of perl.
 
-=item Vstrings are not implemented in the version of perl
+=item Vstrings are not implemented in this version of perl
 
 The version of perl that you are using does not implement Vstrings, to use
 L</isvstring> you will need to use a newer release of perl.
 
+=item isbool is not implemented in this version of perl
+
+The version of perl that you are using does not implement stable boolean
+tracking, to use L</isbool> you will need to use a newer release of perl.
+
 =back
 
 =head1 KNOWN BUGS