docs: add a more detailed explanation to cookies.flushStore() (#39905)

* docs: cookies.flushStore()

Co-authored-by: Spencer17x <[email protected]>

* docs: modify cookies.flushStore()

Co-authored-by: Spencer17x <[email protected]>

* Update docs/api/cookies.md

Co-authored-by: Shelley Vohr <[email protected]>

Co-authored-by: spencer17x <[email protected]>

---------

Co-authored-by: trop[bot] <37223003+trop[bot]@users.noreply.github.com>
Co-authored-by: Spencer17x <[email protected]>

docs/api/cookies.md

@@ -119,4 +119,8 @@ Removes the cookies matching `url` and `name`
 
 Returns `Promise<void>` - A promise which resolves when the cookie store has been flushed
 
-Writes any unwritten cookies data to disk.
+Writes any unwritten cookies data to disk
+
+Cookies written by any method will not be written to disk immediately, but will be written every 30 seconds or 512 operations
+
+Calling this method can cause the cookie to be written to disk immediately.