pam: add note about future deprecation

See #1691 for more information.

modules/misc/news.nix

@@ -1789,6 +1789,23 @@ in
           A new module is available: 'programs.octant'.
         '';
       }
+
+      {
+        time = "2021-01-01T08:51:11+00:00";
+        condition = config.pam.sessionVariables != {};
+        message = ''
+          The option 'pam.sessionVariables' will be deprecated in the future.
+          This is due to PAM 1.5.0 deprecating reading of the user environment.
+          The deprecation will not take place immediately but you may wish to
+          consider alternatives to PAM environment variables.
+
+          See
+
+            https://github.com/nix-community/home-manager/issues/1691
+
+          for discussion.
+      '';
+      }
     ];
   };
 }

modules/misc/pam.nix

@@ -21,6 +21,9 @@ in {
           <refentrytitle>pam_env.conf</refentrytitle>
           <manvolnum>5</manvolnum>
         </citerefentry>.
+        </para><para>
+        Note, this option will become deprecated in the future and its use is
+        therefore discouraged.
       '';
     };
   };