Reformulate section on standalone mode

Since tag 2024.w45/commit f54ca571, standalone mode/--sa is the default and does not exist. Correspondingly, it has been removed from the documentation.

For users reading recent documentation, but running an older version of OAI, there might be a mismatch in that --sa is necessary to run OAI, but not given in the documentation. Provide explanatory text to make users aware of this.