✏️ Clarify docs on CORS (#5627)

docs/en/docs/tutorial/cors.md

@@ -57,7 +57,7 @@ The following arguments are supported:
 * `allow_origins` - A list of origins that should be permitted to make cross-origin requests. E.g. `['https://example.org', 'https://www.example.org']`. You can use `['*']` to allow any origin.
 * `allow_origin_regex` - A regex string to match against origins that should be permitted to make cross-origin requests. e.g. `'https://.*\.example\.org'`.
 * `allow_methods` - A list of HTTP methods that should be allowed for cross-origin requests. Defaults to `['GET']`. You can use `['*']` to allow all standard methods.
-* `allow_headers` - A list of HTTP request headers that should be supported for cross-origin requests. Defaults to `[]`. You can use `['*']` to allow all headers. The `Accept`, `Accept-Language`, `Content-Language` and `Content-Type` headers are always allowed for CORS requests.
+* `allow_headers` - A list of HTTP request headers that should be supported for cross-origin requests. Defaults to `[]`. You can use `['*']` to allow all headers. The `Accept`, `Accept-Language`, `Content-Language` and `Content-Type` headers are always allowed for <a href="https://developer.mozilla.org/en-US/docs/Web/HTTP/CORS#simple_requests" class="external-link" rel="noopener" target="_blank">simple CORS requests</a>.
 * `allow_credentials` - Indicate that cookies should be supported for cross-origin requests. Defaults to `False`. Also, `allow_origins` cannot be set to `['*']` for credentials to be allowed, origins must be specified.
 * `expose_headers` - Indicate any response headers that should be made accessible to the browser. Defaults to `[]`.
 * `max_age` - Sets a maximum time in seconds for browsers to cache CORS responses. Defaults to `600`.