Add config flags and options to the config documentation

[damonfeldman]

We do not currently duplicate the various configuration flags reported by dgraph alpha --help and dgraph zero --help in the documentation.

All or key flags might be added here: https://dgraph.io/docs/deploy/config/

We may simply point people to the "dgraph --help" output, but for find-ability and search indexing, at least the flag names should be in the docs.