add comment about why dlintPreferPrimordials is necessary

tools/lint.js

@@ -51,6 +51,8 @@ async function dlint() {
  }
 }
 
+// `prefer-primordials` has to apply to more limited set of files than the other
+// lint rules, which is why this dedicated function is needed.
 async function dlintPreferPrimordials() {
  const execPath = getPrebuiltToolPath("dlint");
  console.log("prefer-primordials");