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.

Inizia con la guida all’autenticazione collegata e con le guide HOW-TO del repository relative alla risoluzione dei problemi, alle FAQ e alle librerie. Verifica come vengono esposti i token di accesso OAuth2 aggiornati, se devono essere memorizzati e se è disponibile un evento. Il lavoro è completato quando la documentazione descrive chiaramente la procedura prevista per l’aggiornamento, il rilevamento e la memorizzazione.

Scritto dal modello di indicizzazione a partire dal testo della issue.