Update Mattermost integration UI text

What does this MR do?

As per the linked issue, improves the UI Text in the mattermost integration. Also improves the UI text for other integrations that uses the same event notification system.

Moves explanation to the docs, and adds the docs link.

Also externalizes strings for internationalization.

Related to #300609 (closed)

Screenshots (strongly suggested)

Before	After

Does this MR meet the acceptance criteria?

Conformity

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• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
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Security

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