Update doc to recommend that OAuth access token should have an expiration

Problem to solve

Further details

Coming from: #21745 (comment 611083320) - OAuth access tokens do not have an expiry.

Proposal

For this iteration: Update docs to recommend integrations refresh tokens at some interval

Next iteration (not in scope of this issue):

refresh interval should be hard coded to: 2 hours

(GitLab admin can change this configuration - not through the UI)

Who can address the issue

Other links/references