docs/language-frameworks/python: specify how to add non-python runtime deps (#332792)

doc/languages-frameworks/python.section.md

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   and using a `-` as delimiter.
 * Attribute names in `python-packages.nix` should be sorted alphanumerically to
   avoid merge conflicts and ease locating attributes.
+* Non-python runtime dependencies should be added via explicit wrapping or
+  patching (using e.g. `substituteInPlace`), rather than through propagation via
+  `dependencies`/`propagatedBuildInputs`, to reduce clutter in `$PATH`.
 
 This list is useful for reviewers as well as for self-checking when submitting packages.