Clarify how authentication providers work

[julien-f]

When signing in, if the user can be auth (via LDAP for instance), a matching XO user will be created.

[fufroma]

It may also require a "check" button to alert for user existing in XO but no longer existing in the ldap.

[julien-f]

@fufroma this issue is dedicated to the documentation, could you create another issue in https://github.com/vatesfr/xo-web/issues? Thanks :)