ref-manual/variables.rst: update ROOT_HOME documentation

* Since scarthgap [1], the default will be overridden when using
  'systemd' as INIT_MANAGER. Reflect this in the documentation.
* The distro configuration is probably the better place for
  customization, thus at least mention this together with the
  local.conf.
* While at it, drop the probably redundant description on how to
  override weak default variables.
  The example on how to set "/root" is still given indirectly, anyway.

[1] ebafe463 ("systemd: upgrade to 255.1")

(From yocto-docs rev: 50e92009d309fc4ae406174feb8f6578142748cc)

Signed-off-by: Enrico Jörns <ejo@pengutronix.de>
Reviewed-by: Quentin Schulz <quentin.schulz@cherry.de>
Signed-off-by: Antonin Godard <antonin.godard@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

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