Clarify intended procedure for detecting when a new access token has been generated and how/whether to store it

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### Documentation Request

When an access token is expired, performing an API request will automatically use the refresh token to fetch a new access token. However, this does *not* trigger the `on('tokens)` event; this only fires on the first authentication.

What is not explained clearly in the docs then, is how the new access token is meant to be detected so that it can be stored and reused for its validity period. Otherwise the new access token may be lost and users hit 401 Invalid Credentials errors in subsequent requests.

Is the intent that after each API call, we manually check the oAuth2Client for a new access token via `oAuth2Client.credentials.access_token`? I.e., add manual token checking logic after each API call? If this is the case, can explicit instructions be added that something like this is the intended approach?

Or is there some other event similar to `tokens` we can listen for on the `oAuth2Client` that will automatically trigger upon receiving an updated access token? (If not, can that feature be added?)

Perhaps the access token is not meant to be stored at all? Discarded after each request? But then we are fetching new access tokens for every request after the first hour.

Beginne mit dem verlinkten Authentifizierungsleitfaden und den HOW-TO-Leitfäden des Repositorys zur Fehlerbehebung, den FAQs und den Bibliotheken. Kläre, wie aktualisierte OAuth2-Access-Tokens bereitgestellt werden, ob sie gespeichert werden sollten und ob ein Event verfügbar ist. Als erledigt gilt die Aufgabe, wenn die Dokumentation das vorgesehene Verfahren zum Aktualisieren, Erkennen und Speichern klar beschreibt.

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