Surviving Region Failures note should specify node count recommendations

Chuck Droukas (chudro) commented:

In our doc, Surviving Region Failures, there is a note stating:

> In order to survive region failures, you must have added at least 3 database regions

Since this setting is increasing the RF to 5, the note should also detail recommended node counts per cluster/region. Documentation, with the note, can be found here:

https://www.cockroachlabs.com/docs/v21.1/multiregion-overview.html#surviving-region-failures

Jira Issue: DOC-1512