doc: uefi: enhance anti-rollback documentation

To enforce anti-rollback to any older version, dtb must be
always update manually. This should be described in the
documentation.

This commit also adds the recommendation that secure system should not
enable the fdt command because lowest-supported-version
property in device tree can be changed by fdt command.

Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Signed-off-by: Masahisa Kojima <masahisa.kojima@linaro.org>