Refresh token documentation

Please update the documentation on refresh token at
https://docs.authlib.org/en/latest/client/frameworks.html
or
https://docs.authlib.org/en/latest/client/flask.html
as per the suggested solution at
https://stackoverflow.com/questions/62293888/obtaining-and-storing-refresh-token-using-authlib-with-flask

Parameter to specify add in the remote application as per
" .. The first step is register a remote application on the OAuth registry via oauth.register method "
at https://docs.authlib.org/en/latest/client/flask.html
to obtain retrieve get the refresh token

is

authorize_params={'access_type': 'offline'}

```
google = oauth.register(
'google',
# ...
authorize_params={'access_type': 'offline'},
)
```
as per
https://stackoverflow.com/a/62296675

and

'prompt': 'consent' in the client_kwargs parameter in the remote application on the OAuth registry via oauth.register method

if required

`client_kwargs = {'prompt':'consent'}`
as per
https://stackoverflow.com/questions/62293888/obtaining-and-storing-refresh-token-using-authlib-with-flask#comment110182471_62296675

Thank you

Read the OAuth client documentation at docs.authlib.org/en/latest/client/frameworks.html and docs.authlib.org/en/latest/client/flask.html, starting with the oauth.register example. Compare it with the linked Stack Overflow solution and document authorize_params={'access_type': 'offline'} and, when required, client_kwargs={'prompt': 'consent'}. Done means the relevant refresh-token instructions and example are clear in the documentation.

Written by the indexing model from the issue text.