docs(helper/cookie): update getSignedCookie

docs/helpers/cookie.md

@@ -46,12 +46,15 @@ app.get('/signed-cookie', (c) => {
     'cookie_name0'
   )
   deleteCookie(c, 'cookie_name0')
-  // `getSignedCookie` will return `false` for a specified cookie if the signature was tampered with or is invalid
+  // `getSignedCookie` will return `false` for a specified cookie if its signature fails verification
   const allSignedCookies = await getSignedCookie(c, secret)
   // ...
 })
 ```
 
+> [!NOTE]
+> `getSignedCookie` distinguishes two cases. A cookie that has a signature but fails verification returns `false`. A cookie that does not have a valid signature format is treated as not being a signed cookie at all and returns `undefined` — the same as when the cookie is not present. This rule applies both when retrieving a single cookie by name and when retrieving all signed cookies. Since both `false` and `undefined` are falsy, `if (!value)` handles both cases.
+
 ### Cookie Generation
 
 `generateCookie` and `generateSignedCookie` functions allow you to create cookie strings directly without setting them in the response headers.