Improve API Token rotation email

[tsteur]

I got below email that the API token should be rotated:

While API token rotation may be somewhat easy for regular apps it's harder for the mobile app. Created matomo-org/matomo-mobile-2#5468 to make this a bit easier but we should also link to a guide that informs people how to rotate mobile app tokens.

Those tokens can be detected by their name "Matomo Mobile 2"