Add more detail to "Initialize a Cluster" from the cockroach init RFC

[rmloveland]

Initialize a Cluster currently says that cockroach init "performs a one-time initialization", which may be an insufficient level of detail given that:

1. Users are engineers/DBAs who would likely want to know how/why it works a certain way
2. There is a semi-redundant alternative to just use cockroach start with no --join flag, which is very confusing (IMO) and makes it even more important that we be very explicit / informative about why init exists / how it works / what the difference is

To address this we should update Initialize a Cluster with more detail (including design rationale) from the init RFC.

[rmloveland]

Oh, forgot to mention this was prompted by this user question in the forum:
https://forum.cockroachlabs.com/t/what-does-cockroach-init-actually-do/1443

[bdarnell]

There is a semi-redundant alternative to just use cockroach start with no --join flag

I think we should get rid of that alternative: cockroachdb/cockroach#24118. We should at least be using the init command in all of our docs for multi-node clusters.

[jseldess]

Thanks for trigger this thread, @rmloveland. I'm on board with de-emphasizing startup with init, too. We mention it as an option in a few places. In our getting started docs, we should probably be setting expectation right. I actually took this approach in the training.

I'll revisit this if there's time before the release. If not, directly after.

[jseldess]

Closing in favor of #3395.