sbcl: explicitly document the cleanup convention

pkgs/development/compilers/sbcl/2.x.nix

@@ -19,6 +19,9 @@ let
       sha256 = "189gjqzdz10xh3ybiy4ch1r98bsmkcb4hpnrmggd4y2g5kqnyx4y";
     };
 
+    # The loosely held nixpkgs convention for SBCL is to keep the last two
+    # versions.
+    # https://github.com/NixOS/nixpkgs/pull/200994#issuecomment-1315042841
     "2.3.2" = {
       sha256 = "sha256-RMwWLPpjMqmojHoSHRkDiCikuk9r/7d+8cexdAfLHqo=";
     };