User feedback on REALTIME table update for topic modification

Following Slack discussion : https://apache-pinot.slack.com/archives/C011C9JHN7R/p1622809155406800

Changing the topic a REALTIME table is consuming from is not possible as for now.
-> `stream.kafka.topic.name`

However, a table definition update is fully accepted without warnings.

We should provide some user feedback regarding this limitation.

1. In the docs: I'll open a PR.
2. At the API level: We could either warn or fully reject the table update in such instances.

Start by locating the API handling REALTIME table definition updates and the validation of stream.kafka.topic.name. Review the existing documentation for REALTIME tables and the linked Slack discussion for context. Done means the unsupported topic modification receives clear user feedback at the API level and the limitation is documented.

Written by the indexing model from the issue text.