Improve email notification issues documentation and UI text

Problem to solve

When reviewing notifications-related UI text, I identified some problems with the current doc at https://docs.gitlab.com/ee/user/profile/notifications.html

!187185 (merged) We don't explain in the UI or docs when the "issue is due soon" email is sent.

From my exploration, it looks like this email is always sent the day before the due date, so let's say it in:
- Docs: https://docs.gitlab.com/ee/user/project/issues/due_dates.html, https://docs.gitlab.com/ee/user/profile/notifications.html
- UI: custom notification settings: due soon -> due tomorrow (related: !154449 (comment 1933417843) and #451899 (closed))
- Email content: due soon -> due tomorrow https://gitlab.com/gitlab-org/gitlab/-/blob/master/app/views/notify/issue_due_email.html.haml#L2
!187193 (merged) The Watch level currently says: Receive notifications for any activity.

However, @engwan pointed out that the following events are disabled for Watchers (source in code):
- push_to_merge_request
- issue_due
- success_pipeline
- approver
Related:
- #497866 (comment 2173867382)
- !170405 (comment 2181700271)
!187571 (merged) It's unclear when "Added as MR approver" notification is sent.

Answer: when an MR you're eligible to approve, via CODEOWNERS, but maybe not only, is created. The email isn't necessarily sent immediately.

Related: #451899 (comment 1830701415)

Additionally, email text could be improved at the same time: #451899 (comment 2431710413)
!187701 (merged) Improve descriptions for notification events in https://docs.gitlab.com/api/notification_settings/#valid-notification-levels