Clarify that getting emails only applies to some enterprise users

What does this MR do?

It confuses both customers and Support Engineers that the documentation claims this feature is available, when it isn't yet for many enterprise users.

See the following Zendesk tickets as examples:

• https://gitlab.zendesk.com/agent/tickets/471076
• https://gitlab.zendesk.com/agent/tickets/471816
• https://gitlab.zendesk.com/agent/tickets/457767

...and many more.

This MR clarifies that the feature is only available for enterprise users that were provision for the group. It's planned to make it available for all enterprise users in 16.7.

Related issues

#391453 (closed)

Related MRs

!137322 (merged)

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