Explain why --only-download checks are there

cabal-install/src/Distribution/Client/ProjectBuilding.hs

@@ -653,6 +653,14 @@ rebuildTarget verbosity
               sharedPackageConfig
               plan rpkg@(ReadyPackage pkg)
               pkgBuildStatus
+    -- Technically, doing the --only-download filtering only in this function is
+    -- not perfect. We could also prune the plan at an earlier stage, like it's
+    -- done with --only-dependencies. But...
+    --   * the benefit would be minimal (practically just avoiding to print the
+    --     "requires build" parts of the plan)
+    --   * we currently don't have easy access to the BuildStatus of packages
+    --     in the pruning phase
+    --   * we still have to check it here to avoid performing successive phases
     | buildSettingOnlyDownload buildSettings = do
         case pkgBuildStatus of
           BuildStatusDownload ->