chore(compartment-mapper): add note about package policy handling

Author: boneskull

packages/compartment-mapper/src/policy.js

@@ -155,10 +155,13 @@ export const makePackagePolicy = (label, { policy } = {}) => {
       };
     } else if (label === ENTRY_COMPARTMENT) {
       packagePolicy = policy.entry;
+      // If policy.entry is `undefined`, we return `undefined` which is
+      // equivalent to "allow everything".
       return packagePolicy;
     } else if (label) {
       packagePolicy = policy.resources?.[label];
     }
+    // An empty object for a package policy is equivalent to "allow nothing"
     return packagePolicy ?? create(null);
   }
   return undefined;