[faq] note about updating config users

First comment:

If a user already exists in the backing bucket, the user config in config.json will not be applied.

This is to prevent updates to users via the ADMIN REST api being blown away.

But it can be a little confusing for devs, who might change the "admin_channels" for a user and not see the users doc visibility change after a restart.

To pick up user changes in the config.json, the user record must be removed from the backing bucket before restarting SG.

see other comments: https://github.com/couchbaselabs/couchbase-mobile-portal/issues/387