docs: added comment on sveltekit plugin integration (#3899)

Added comment to the user to place cookie setting plugin as the last item in the array similar to Next.js.

If not, will deal with problems on other plugins such as twoFactor

docs/content/docs/integrations/svelte-kit.mdx

@@ -62,7 +62,7 @@ import { getRequestEvent } from "$app/server";
 
 export const auth = betterAuth({
   // ... your config
-  plugins: [sveltekitCookies(getRequestEvent)],
+  plugins: [sveltekitCookies(getRequestEvent)], // make sure this is the last plugin in the array
 });
 ```