Merge pull request #1614 from Tyil/update-module-meta6

Remove link to outdated META6 spec

doc/Language/modules.pod6

@@ -560,10 +560,13 @@ lib
     C<%?RESOURCES> Hash indexed on the name provided.
 
     The L<Test::META module | https://github.com/jonathanstowe/Test-META/>
-    can help you check the correctness of the META.info file.  See the full
-    L<META specification | https://design.perl6.org/S22.html#META6.json> for
-    more details as well as further options available for use in
-    C<META6.json> files.
+    can help you check the correctness of the META.info file.
+
+    There are more fields described in the L<META design documents |
+    https://design.perl6.org/S22.html#META6.json>, but not all of these are
+    implemented by existing package managers. Hence you should stick to the
+    fields described in the above example block to ensure compatibility with
+    existing package managers.
 
     =end item