Fix LevelDB yaml unmarshall error and update documentation

[endoze]

In order to ensure the project correctly validates authentication methods in the config file that can use a token db, this commit modifies the logic to ensure we check if all of the token methods are nil before outputting an error. Previously, if the local filesystem token db method was not nil and the other two (redis and google cloud storage) were nil, we would return an error and the config would be considered invalid.

Additionally, this commit documents the correct LevelDB settings for the config file. Prior to the addition of Bcrypt hashing cost, just using token_db was acceptable in the configuration along with a string representing the path. Given the swap to a struct instead of a string, we need to update documentation concerning this.