docs: document detach as the transport option it is

[tas50]

detach was listed under Driver configuration → Running the container, where setting it does nothing at all.

The only place config[:detach] is read is CliHelper#build_exec_command:

def build_exec_command(state, command)
  cmd = "exec"
  cmd << " -d" if config[:detach]

and the only thing that reaches that is the transport's container. The driver constructs a container too, but only ever calls create and destroy on it, neither of which runs an exec — so a detach: true under driver: is read by nothing.

Its own description gave it away: it talks about provisioner commands and about kitchen login, both of which are the transport's business.

What changed

• detach moved to the transport table, where setting it has the documented effect.
• A pointer left behind in the driver section for anyone who looks where it used to be, since the sentence about the container always being started detached is still worth saying there.
• build_tempdir corrected. It said "relative to build_context"; it is resolved against the working directory (Pathname.pwd + config[:build_tempdir]), and dockerfile_path then makes the -f argument relative to the working directory too, which is what constrains where it can be.

Documentation only — no behaviour change. I checked the rest of the option tables against default_config and the config keys lib/ actually reads while I was in here; everything else lines up, including the defaults.

Verification

$ npx markdownlint-cli2 README.md
Summary: 0 issues in 0 files

$ bundle exec rake test
360 examples, 0 failures

$ cookstyle --chefstyle          # Cookstyle 9.0.0 / RuboCop 1.90.0
44 files inspected, no offenses detected