cockroach start Doc Update

[bdarnell]

Re: cockroach start

Issue Description

The warning that says "However, in a 3-node cluster with multiple stores per node..." is obsolete. We support rebalancing between stores in a 3-node cluster since cockroachdb/cockroach#51567 (which I believe landed in 21.1)

Suggested Resolution

Remove this warning from the docs in 21.1 and future versions.

[jseldess]

The change was actually made in 20.2.0-beta1, so we need to remove the warning for 20.2 and later: https://www.cockroachlabs.com/docs/releases/v20.2.0-beta.1#performance-improvements (sixth bullet).