Add differently worded reference to S22

There seems to be very little response from the current zef maintainer
to improve the situation at all, so I'll just reword the docs better. It
saddens me that he seems uninterested to solve the root problem, though.

Author: Tyil

doc/Language/modules.pod6

@@ -562,6 +562,12 @@ lib
     The L<Test::META module | https://github.com/jonathanstowe/Test-META/>
     can help you check the correctness of the META.info file.
 
+    There are more fields described in the L<META specification |
+    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 compatability with
+    existing package managers.
+
     =end item
 
     =item Put your project under git version control if you haven't done so