cockroach start Doc Update

[keith-mcclellan]

Re: cockroach start
cockroach start page still doesn't describe how and when to set the --sql-addr flag to use a separate SQL port from the RPC port.

Issue Description

cockroachdb/cockroach#30828
https://www.cockroachlabs.com/docs/releases/v19.2.0-beta.20190930.html

Suggested Resolution

Add the description of this flag to the docs for cockroach start

[jseldess]

This is a known docs gap from 19.2. It's on @Amruta-Ranade's backlog: #4771. Amruta, we should look at adding docs on the flags at a minimum, even if we don't adapt any examples/tutorials.