clarify behavior of *-addr flags in cockroach start

Ryan Kuo (taroface) commented:

Our `cockroach start` docs can include more details on how the various `*-addr` flags behave according the port and hostname/IP they're given, e.g.:

> What is the behavior of an --XXX-addr flag that has only a port, and no hostname/IP (ex: --sql-addr above)? Does it listen/advertise on all IPs on that port?
> What is the behavior … that only has the hostname/IP and no port (ex: --advertise-addr above)? Does it listen/advertise on that hostname/IP on the default port?
> For join if only the hostname/IP is specified, does it try to join at the default port 26257 (ex: --join above)?

(Per @bdarnell, yes to all the above)

Thread: https://cockroachlabs.slack.com/archives/C65RYUY3Z/p1600706082015600

cc @Amruta-Ranade

Jira Issue: DOC-778