lib: Document status of deprecated.nix and move it

This is all I could find after co-maintaining lib for a long time.
I've had the fortune of basically not really noticing this file,
because it has had very few interactions until the confusion in
NixOS/nixpkgs#304277

It seems to be a state of limbo, which would be nice to resolve
(with great care), but this is not urgent, and first we should
document its status.

lib/default.nix

@@ -47,7 +47,7 @@ let
     # misc
     asserts = callLibs ./asserts.nix;
     debug = callLibs ./debug.nix;
-    misc = callLibs ./deprecated.nix;
+    misc = callLibs ./deprecated/misc.nix;
 
     # domain-specific
     fetchers = callLibs ./fetchers.nix;