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: f1f92f0a5a881c45ffecf9cf9bd070c992aff35a)

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>
(cherry picked from commit 50e92009d309fc4ae406174feb8f6578142748cc)
Signed-off-by: Antonin Godard <antonin.godard@bootlin.com>
Signed-off-by: Steve Sakoman <steve@sakoman.com>

documentation/ref-manual/variables.rst

@@ -7385,17 +7385,12 @@ system and gives an overview of their function and contents.
          prefer to have a read-only root filesystem and prefer to keep
          writeable data in one place.
 
-      You can override the default by setting the variable in any layer or
-      in the ``local.conf`` file. Because the default is set using a "weak"
-      assignment (i.e. "??="), you can use either of the following forms to
-      define your override::
+      When setting ``INIT_MANAGER = systemd``, the default will be set to::
 
-         ROOT_HOME = "/root"
          ROOT_HOME ?= "/root"
 
-      These
-      override examples use ``/root``, which is probably the most commonly
-      used override.
+      You can also override the default by setting the variable in your distro
+      configuration or in the ``local.conf`` file.
 
    :term:`ROOTFS`
       Indicates a filesystem image to include as the root filesystem.