docs: fix documentation of signedCookie return value

fixes #32

README.md

@@ -42,7 +42,7 @@ Given an object, this will iterate over the keys and call `JSONCookie` on each v
 
 ### cookieParser.signedCookie(str, secret)
 
-Parse a cookie value as a signed cookie. This will return the parsed unsigned value if it was a signed cookie and the signature was valid, otherwise it will return the passed value.
+Parse a cookie value as a signed cookie. This will return the parsed unsigned value if it was a signed cookie and the signature was valid. If the value was not signed, the original value is returned. If the value was signed but the signature could not be validated, `false` is returned.
 
 The `secret` argument can be an array or string. If a string is provided, this is used as the secret. If an array is provided, an attempt will be made to unsign the cookie with each secret in order.