Get attribute-based access control info into docs

[shainaraskas]

At the end of the file there's a section about attribute-based access control that ends with:

For more information, see Document-level attribute-based access control with X-Pack 6.1.

I don't know if linking to such an old blog post is a good idea if we now have a lot of docs about Document Level Security... for example we could link to wherever this page is mapped: https://www.elastic.co/guide/en/elasticsearch/reference/current/es-dls-overview.html

Originally posted by @eedugon in #340 (comment)

[shainaraskas]

^ linked page is for connectors only. need to assess the blog and the connectors page and create something in the auth section