Update Slack-Import.md

doc/help/Slack-Import.md

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 ### Slack Import
 
-*Note: As a SaaS service, Slack is able to change its export format quickly. If you encounter issues not mentioned in the documentation below, please let us know by [filing an issue](https://github.com/mattermost/platform/issues).*
+*Note: As a proprietary SaaS service, Slack is able to change its export format quickly and without notice. If you encounter issues not mentioned in the documentation below, please let us know by [filing an issue](https://github.com/mattermost/platform/issues).*
 
 #### Usage