Update with "Private to my company" notified to tagged user, also if not assigned to the company - More meaningful representation in the UI

[#1]

Description - If an update / a comment is created using the checkbox Private to my company and a user is tagged, who is not assigned to the originators company, this user gets notified on this update. The user will also be able to read the update / note text in the notification email / notifications, but he/she is not able to see the update in the UI.
This can be very confusion for the originator of the comment / update as well as the tagged user.

Why is this feature important to you - To avoid any misunderstandings.

How would you like the feature to work - There are multiple ways, how this could be handled.:
- information in the update / comment editor, if the checkbox Private to my company is active and a person from a different company is tagged
- denial of tagging users who are not assigned to the company as long as the checkbox is active

- information in the update stream of tagged user, that there is an update, which was set to private

- different functionality in the background -> User get's notified, but does not see the note text, or is able to ask the originator for permissions like it is possible on other objects

Current Behaviour - as described above