doc/by-name: explain motivation for transition when custom args are used

pkgs/by-name/README.md

@@ -92,7 +92,7 @@ though this is not required and must not be expected.
 
 Note that definitions in `all-packages.nix` with custom arguments should not be removed.
 That is a backwards-incompatible change because it changes the `.override` interface.
-Such packages may still be moved to `pkgs/by-name` however, while keeping the definition in `all-packages.nix`.
+Such packages may still be moved to `pkgs/by-name` however, in order to avoid the slightly superficial choice of directory / category in which the `default.nix` file was placed, but please keep the definition in `all-packages.nix` using `callPackage`.
 See also [changing implicit attribute defaults](#changing-implicit-attribute-defaults).
 
 ## Limitations