Clarify the definition of "Enterprise User"

Why is this change being made?

We strive to come to the state when "Enterprise user" term == user with provisioned_by_group_id that matches the group's ID. The GitLab implementation reflects this definition.

In gitlab-org&9675 (closed) we want to create the automatic process that will [mark a user as an Enterpriser user == set provisioned_by_group_id] if they match the following condition:

1. The user's email has a domain that is owned by the company of the paid group, _and_ the user account meets _one of_ the following conditions:
  - was created 2021-02-01 or later.
  - has a SAML or SCIM identity tied to the organization's group.
  - is a member of the organization's group, where the subscription was purchased or renewed 2021-02-01 or later.

This MR is intended to clarify the definition of the "Enterprise User" term: If a user has a provisioned_by_group_id value that is the same as the organization's group's ID, they are an Enterprise User.

Related to gitlab-org/gitlab#385785 (closed)

Author Checklist

• Provided a concise title for this Merge Request (MR)
• Added a description to this MR explaining the reasons for the proposed change, per say why, not just what
  • Copy/paste the Slack conversation to document it for later, or upload screenshots. Verify that no confidential data is added, and the content is SAFE
• Assign reviewers for this MR to the correct Directly Responsible Individual/s (DRI)
  • If the DRI for the page/s being updated isn’t immediately clear, then assign it to one of the people listed in the Maintained by section on the page being edited
  • If your manager does not have merge rights, please ask someone to merge it AFTER it has been approved by your manager in #mr-buddies
  • The when to get approval handbook section explains the workflow in more detail
• If the changes affect team members, or warrant an announcement in another way, please consider posting an update in #whats-happening-at-gitlab linking to this MR
  • If this is a change that directly impacts the majority of global team members, it should be a candidate for #company-fyi. Please work with internal communications and check the handbook for examples.

---