List the privileges that the citus user can('t) grant on Cloud

## Communication goals (e.g. detailed howto vs orientation)

@colton-citus reports that someone wanted to run `GRANT CREATE ON SCHEMA` and was unable.

We should include a table of which of the [grant statements](https://www.postgresql.org/docs/current/static/sql-grant.html) is available to the "citus" user since he's not a superuser.

### Good locations for content in docs structure

http://docs.citusdata.com/en/v7.5/cloud/security.html#users-and-permissions