mailman/python: remove obsolete almebic override, add explanation why this empty overlay should be kept

pkgs/servers/mail/mailman/python.nix

@@ -3,10 +3,21 @@
 python3.override {
   packageOverrides = lib.composeExtensions
     (self: super: {
-      # does not find tests
-      alembic = super.alembic.overridePythonAttrs (oldAttrs: {
-        doCheck = false;
-      });
+      /*
+        This overlay can be used whenever we need to override
+        dependencies specific to the mailman ecosystem: in the past
+        this was necessary for e.g. psycopg2[1] or sqlalchemy[2].
+
+        In such a large ecosystem this sort of issue is expected
+        to arise again. Since we don't want to clutter the python package-set
+        itself with version overrides and don't want to change the APIs
+        in here back and forth every time this comes up (and as a result
+        force users to change their code accordingly), this empty overlay
+        is kept on purpose.
+
+        [1] 72a14ea563a3f5bf85db659349a533fe75a8b0ce
+        [2] f931bc81d63f5cfda55ac73d754c87b3fd63b291
+      */
     })
     overlay;
 }