samples: boards: nrf: system_off: update readme

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This sample can be used for basic power measurement and as an example of

deep sleep on Nordic platforms.  The functional behavior is:

* Busy-wait for 2 seconds

* Sleep for 2 seconds without device power control

* Sleep for 2 seconds after turning UART off, which reduces current draw

  by a couple hundred uA

* Turn the system off after enabling wakeup through a button press

A power monitor will be able to distinguish among these states.

This sample also demonstrates the use of a :c:func:`SYS_INIT()` call to

disable the deep sleep functionality before the kernel starts, which

prevents the board from powering down during initialization of drivers

that use unbounded delays to wait for startup.

deep sleep on Nordic platforms.

RAM Retention

=============

This application uses nRF51 DK or nRF52 DK board for the demo.

Building, Flashing and Running

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.. zephyr-app-commands::

   :zephyr-app: samples/boards/nrf/system_off

   :board: nrf52dk_nrf52832

   :goals: build flash

   :compact:

Running:

1. Open UART terminal.

2. Power Cycle Device.

3. Device will demonstrate two activity levels which can be measured.

4. Device will demonstrate long sleep at minimal non-off power.

5. Device will turn itself off using deep sleep state 1.  Press Button 1

   to wake the device and restart the application as if it had been

   powered back on.

Sample Output

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=============

nRF52 core output

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   *** Booting Zephyr OS build v2.3.0-rc1-204-g5f2eb85f728d  ***

   nrf52dk_nrf52832 system off demo

   Busy-wait 2 s

   Busy-wait 2 s with UART off

   Sleep 2 s

   Sleep 2 s with UART off

   Entering system off; press BUTTON1 to restart

   Entering system off; press sw0 to restart