packaging/dependency: Clarify darwinMinVersion

(cherry picked from commit 4be92e7b82)

packaging/dependencies.nix

@@ -26,6 +26,9 @@ let
   # Despite the use of the 10.13 deployment target here, the aligned
   # allocation function Clang uses with this setting actually works
   # all the way back to 10.6.
+  # NOTE: this is not just a version constraint, but a request to make Darwin
+  #       provide this version level of support. Removing this minimum version
+  #       request will regress the above error.
   darwinStdenv = pkgs.overrideSDK prevStdenv { darwinMinVersion = "10.13"; };
 
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