docs: explain when custom llms.txt files are public on authenticated sites

ai/llmstxt.mdx

@@ -121,3 +121,9 @@ Mintlify automatically hosts an `llms-full.txt` file at the root of your project
 To add a custom `llms.txt` or `llms-full.txt` file, create an `llms.txt` or `llms-full.txt` file at the root of your project. Adding a custom file overrides the automatically generated file of the same name. If you delete a custom file, Mintlify restores the automatically generated file.
 
 Your custom `llms.txt` or `llms-full.txt` file must have a site title as an H1 heading. Other content is optional. See [Format](https://llmstxt.org/#format) in the `llms.txt` specification for more information on optional sections and best practices.
+
+<Note>
+On sites that use [authentication](/deploy/authentication-setup), custom `llms.txt` and `llms-full.txt` respect authentication.
+
+If a custom file links to any page that ins't public, only authenticated users can access the file. Anonymous visitors receive the automatically generated `llms.txt` or `llms-full.txt`, which only include publicly available links.
+</Note>