fix: Update return types and comments

src/sources/apple_pay.test.ts

@@ -23,7 +23,7 @@ describe('Sources', () => {
           ApplePaySession: undefined,
         },
         async () => {
-          expect(await getApplePayState()).toBe(ApplePayState.NoAPI)
+          expect(getApplePayState()).toBe(ApplePayState.NoAPI)
         },
       )
     })

src/sources/apple_pay.ts

@@ -14,6 +14,10 @@ export const enum ApplePayState {
   NotAvailableInFrame = -3,
 }
 
+/**
+ * The return type is a union instead of the enum, because it's too challenging to embed the const enum into another
+ * project. Turning it into a union is a simple and an elegant solution.
+ */
 export default function getApplePayState(): 0 | 1 | -1 | -2 | -3 {
   const { ApplePaySession } = window
 
@@ -28,14 +32,9 @@ export default function getApplePayState(): 0 | 1 | -1 | -2 | -3 {
   }
 }
 
-/**
- * The return type is a union instead of the enum, because it's too challenging to embed the const enum into another
- * project. Turning it into a union is a simple and an elegant solution.
- *
- * Warning for package users:
- * This function is out of Semantic Versioning, i.e. can change unexpectedly. Usage is at your own risk.
- */
-export function getStateFromError(error: unknown): -2 | -3 {
+export function getStateFromError(
+  error: unknown,
+): Exclude<ApplePayState, ApplePayState.Disabled | ApplePayState.Enabled | ApplePayState.NoAPI> {
   if (error instanceof Error) {
     // See full expected error messages in the test
     if (error.name === 'InvalidAccessError') {