doc: add explanatory code comment

let

  mergeInputs = name:

    (attrs.${name} or [ ]) ++

    # 1. get all `{build,nativeBuild,...}Inputs` from the elements of `inputsFrom`

    # 2. since that is a list of lists, `flatten` that into a regular list

    # 3. filter out of the result everything that's in `inputsFrom` itself

    # this leaves actual dependencies of the derivations in `inputsFrom`, but never the derivations themselves

    (lib.subtractLists inputsFrom (lib.flatten (lib.catAttrs name inputsFrom)));

  rest = builtins.removeAttrs attrs [