improve the doc for nativeauthenticator

[vitrinite]

Proposed change

improve the doc for nativeauthenticator.

Alternative options

Jupyterhub is great, but the FirstUseAuthenticator is not so nice. So I decide to switch to nativeauthenticator that seems to better correspond to what I need.
But the first part of the documentation (here http://tljh.jupyter.org/en/latest/howto/auth/nativeauth.html#enabling-the-authenticator) use tljh-config to configure jupyterhub and the last part for optionnal features use a config file (here https://native-authenticator.readthedocs.io/en/latest/options.html)
it is not clear if the two methods are compatible.
I do not understand where the setting configure with tljh-config are stored.
To use the configure file you have to manually launch jupyterhub, which seems not like a good practice.
The file generate by the command jupyterhub --generate-config, have nothing for the nativeauthenticator.

Who would use this feature?

yes to install the natiauthenticator

(Optional): Suggest a solution

generate more consistent documentation, the one here https://native-authenticator.readthedocs.io/en/latest/options.html seems less up-to-date that the rest of the documentation

[consideRatio]

Thanks for your feedback! I'll go ahead and close this issue in favor of tracking it in #264 and #731.