[io grant] Document IO::Spec::* don't do any validation

Addresses RT#130900:
https://rt.perl.org/Ticket/Display.html?id=130900

doc/Type/IO/Spec.pod6

@@ -18,6 +18,10 @@ B<NOTE:> the C<IO::Spec::*> classes provide low-level path operations. Unless
 you're creating your own high-level path manipulation routines, you don't
 need to use C<IO::Spec::*>. Use L«C<IO::Path>|/type/IO::Path» instead.
 
+B<NOTE2:> no special validation is done by these classes (e.g. check whether
+path contains a NUL byte). It is the job of higher-level classes, like
+L«C<IO::Path>|/type/IO::Path», to do that.
+
 =head1 Methods
 
 =end pod

doc/Type/IO/Spec/Cygwin.pod6

@@ -13,6 +13,10 @@ B<NOTE:> the C<IO::Spec::*> classes provide low-level path operations. Unless
 you're creating your own high-level path manipulation routines, you don't
 need to use C<IO::Spec::*>. Use L«C<IO::Path>|/type/IO::Path» instead.
 
+B<NOTE2:> no special validation is done by these classes (e.g. check whether
+path contains a NUL byte). It is the job of higher-level classes, like
+L«C<IO::Path>|/type/IO::Path», to do that.
+
 =head1 Methods
 
 =head2 method abs2rel

doc/Type/IO/Spec/QNX.pod6

@@ -13,6 +13,10 @@ B<NOTE:> the C<IO::Spec::*> classes provide low-level path operations. Unless
 you're creating your own high-level path manipulation routines, you don't
 need to use C<IO::Spec::*>. Use L«C<IO::Path>|/type/IO::Path» instead.
 
+B<NOTE2:> no special validation is done by these classes (e.g. check whether
+path contains a NUL byte). It is the job of higher-level classes, like
+L«C<IO::Path>|/type/IO::Path», to do that.
+
 =head1 Methods
 
 =head2 method canonpath

doc/Type/IO/Spec/Unix.pod6

@@ -13,6 +13,10 @@ B<NOTE:> the C<IO::Spec::*> classes provide low-level path operations. Unless
 you're creating your own high-level path manipulation routines, you don't
 need to use C<IO::Spec::*>. Use L«C<IO::Path>|/type/IO::Path» instead.
 
+B<NOTE2:> no special validation is done by these classes (e.g. check whether
+path contains a NUL byte). It is the job of higher-level classes, like
+L«C<IO::Path>|/type/IO::Path», to do that.
+
 =head1 Methods
 
 =head2 method abs2rel

doc/Type/IO/Spec/Win32.pod6

@@ -14,6 +14,10 @@ B<NOTE:> the C<IO::Spec::*> classes provide low-level path operations. Unless
 you're creating your own high-level path manipulation routines, you don't
 need to use C<IO::Spec::*>. Use L«C<IO::Path>|/type/IO::Path» instead..
 
+B<NOTE2:> no special validation is done by these classes (e.g. check whether
+path contains a NUL byte). It is the job of higher-level classes, like
+L«C<IO::Path>|/type/IO::Path», to do that.
+
 =head1 Methods
 
 =head2 method basename