Update sections that talk about "adding more master nodes"

A prospective user asked about "adding more master nodes" today.

We have two sections in our documentation talk about adding master nodes. We should clarify our language in these sections to make sure that this isn't production-ready. We could either cover this type of advice in a separate section (more experimental) or hint at the upcoming MX solution.

We currently talk about adding master nodes in the following sections:
- Cluster Management > Scaling out your cluster > Adding a master (second paragraph)
- Scaling out data ingestion > Hash distributed tables > Masterless Citus (the example in this section is a bit out of sync with the previous sections)