Bot users don't count towards subscription

What does this MR do and why?

According to the Billable Users section of our documentation:

Every user is considered a billable user, with the following exceptions:

...

GitLab-created service accounts:

• Ghost User.
• Bots such as:
  • Support Bot.
  • Bot users for projects.
  • Bot users for groups.
  • Other internal users.

I've updated the documentation to be a bit more clear to reflect the above.

How to set up and validate locally

Check the text manually.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.