buildPythonPackage: document why we always propagate python

python applications should not propagate python in PATH but we do it
nevertheless to avoid rebuilds see https://github.com/NixOS/nixpkgs/issues/170887
for an explanation.

pkgs/development/interpreters/python/mk-python-derivation.nix

@@ -148,7 +148,12 @@ let
 
     buildInputs = buildInputs ++ pythonPath;
 
-    propagatedBuildInputs = propagatedBuildInputs ++ [ python ];
+    propagatedBuildInputs = propagatedBuildInputs ++ [
+      # we propagate python even for packages transformed with 'toPythonApplication'
+      # this pollutes the PATH but avoids rebuilds
+      # see https://github.com/NixOS/nixpkgs/issues/170887 for more context
+      python
+    ];
 
     inherit strictDeps;