Clarify that "Watch" doesn't include all notifications

What does this MR do and why?

Clarify that "Watch" doesn't include all notifications:

• In the docs
• In the notification settings

References

• Related to Update `Watch` events to include all events (#501083)
• Implements Improve email notification issues documentation... (#465347 - closed)

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. On the left sidebar, select your avatar.
2. Select Preferences.
3. On the left sidebar, select Notifications.
4. Select Global notification level, and observe Watch.

MR acceptance checklist

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