doc: add mention of nRF Cloud Multi-service sample to CoAP library

Adds the nRF Cloud multi-service library to the Samples using
the library section of the nRF Cloud CoAP library.

Signed-off-by: Wille Backman <wille.backman@nordicsemi.no>

doc/nrf/libraries/networking/nrf_cloud_coap.rst

@@ -97,6 +97,7 @@ Samples using the library
 The following |NCS| samples use this library:
 
 * :ref:`modem_shell_application`
+* :ref:`nrf_cloud_multi_service`
 
 Limitations
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