dts: Add a comment about CONFIG_OF_EMBED being for local use

This comment from README.fdt-control did not end up in the Kconfig, which
is what most people will see. Add it with a few tweaks.

Signed-off-by: Simon Glass <sjg@chromium.org>
Acked-by: Michal Simek <michal.simek@xilinx.com>