[llvm][docs] Update "Adding a Builder" docs

* Move step 8 to later, after worker credentials have
  been added to the buildmaster.
* Added command for starting the worker, in addition
  to creating the worker. The latter only sets up the
  directories.
* Noted that in step 6, it is expected that you get a
  refused connection.
* Stated that the connection should be tried once,
  and the worker then stopped.

We could mention that repeated connections with invalid
credentials will result in an IP ban, but it's probably
detail people don't need here.

If it did happen, then you would not know until you tried
the later steps. At which point you are already in contact
with Galina, who is the person who would help you with that
issue in any case.

Reviewed By: gkistanova

Differential Revision: https://reviews.llvm.org/D148913