offlineimap.conf: improve documentation for oauth2

Signed-off-by: Nicolas Sebrecht <nicolas.s-dev@laposte.net>

offlineimap.conf

@@ -804,8 +804,15 @@ remoteuser = username
 #         python python/oauth2.py --user=YOUR_EMAIL --client_id=YOUR_CLIENT_ID
 #            --client_secret=YOUR_CLIENT_SECRET --refresh_token=REFRESH_TOKEN
 #
-#oauth2_refresh_token = REFRESH_TOKEN
+# Access tokens have limited lifetimes. If you need access beyond the lifetime
+# of a single access token, you should use a refresh token. A refresh token
+# allows offlineimap to obtain new access tokens.
+#
+# If you want to use a refresh token, make sure you disabled/removed any
+# oauth2_access_token option.
+#
 #oauth2_access_token = ACCESS_TOKEN
+#oauth2_refresh_token = REFRESH_TOKEN
 
 ########## Passwords