doc: Explain cabalSdist test

  localFromCabalSdist = haskellPackages.buildFromCabalSdist localRaw;

  # NOTE: ./local refers to the "./." path in `./local/generated.nix`.

  # This test makes sure that localHasNoDirectReference can actually fail if

  # it doesn't do anything. If this test fails, either the test setup was broken,

  # or Haskell packaging has changed the way `src` is treated in such a way that

  # either the test or the design of `buildFromCabalSdist` needs to be reconsidered.

  assumptionLocalHasDirectReference = runCommand "localHasDirectReference" {

    drvPath = builtins.unsafeDiscardOutputDependency localRaw.drvPath;

  } ''

mkDerivation {

  pname = "local";

  version = "0.1.0.0";

  src = ./.;

  src = ./.; # also referred to as ./local in the test; these are the same path constants

  isLibrary = false;

  isExecutable = true;

  executableHaskellDepends = [ base ];