Add note about Index Access feature

[jpipkin1]

Purpose of this pull request

This pull request adds a note to this article: https://help.sumologic.com/docs/manage/users-roles/roles/create-manage-roles/

Select the type of change

• Minor Changes - Typos, formatting, slight revisions
• Update Content - Revisions, updating sections
• New Content - New features, sections, pages, tutorials
• Site and Tools - .clabot, version updates, maintenance, dependencies, new packages for the site (Docusaurus, Gatsby, React, etc.)

Ticket (if applicable)

Request in the #rbac-for-indexes Slack channel