Merge branch 'eduardosanz/lifetime-access-token-documentation' into 'master'

Fix inconsistencies with `expires_at` access token

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/175995



Merged-by: Jon Glassman <jglassman@gitlab.com>
Approved-by: Jon Glassman <jglassman@gitlab.com>
Reviewed-by: Isaac Durham <idurham@gitlab.com>
Co-authored-by: Eduardo Sanz García <esanz-garcia@gitlab.com>