Misleading text in access screen

[mpivchev]

Ref: https://support.nextcloud.com/#ticket/zoom/68778

The access screen misleadingly states that admins have access to user's data, and while that may be true in some cases, by default it's not.
We've received reports that users think admins have complete access to user's data, so we must correct what's written.

Something like this can be added to the heading:

The following users are designated administrators and may potentially have access to your data through system storage or backups. However, they do not have easy access to your data via the web interface or client applications.