doc: nrf_modem: add documentation about band 4 support

Add documentation about band 4 support.
Minor updates.

Signed-off-by: Emanuele Di Santo <[email protected]>

Author: lemrey

nrf_modem/doc/dectphy.rst

@@ -72,7 +72,12 @@ Before initializing the DECT PHY interface of the Modem library, the application
 Afterwards, the application can initialize the DECT PHY interface by calling the :c:func:`nrf_modem_dect_phy_init` function.
 Upon successful initialization, both the DECT PHY interface and DECT NR+ physical layer in the modem are ready for operation.
 
-The very first time the DECT PHY interface is initialized on a nRF9131 SiP, the DECT NR+ PHY firmware locks the calibration data preventing any changes to it.
+On nRF9151 devices only, the application can configure band 4 support during initialization by setting the :c:member:`nrf_modem_dect_phy_init_params.band4_support` field to 1.
+
+.. important::
+   When operating on band 4, the carries outside of the [525, 551] range shall not be used as they interfere with other radio devices, including LTE devices.
+
+On nRF9131 devices only, the DECT NR+ PHY firmware locks the calibration data the very first time the DECT PHY interface is initialized.
 
 The DECT PHY interface can be de-initialized, which in turn de-initializes the physical layer in the modem, cancelling all scheduled operations.
 
@@ -178,16 +183,19 @@ Channel frequency
 -----------------
 
 The DECT radio band is divided into different channels, as described in chapter 5.2 of `ETSI TS 103 636-2`_.
+
 The absolute radio channel frequency used for transmission is controlled by the :c:member:`nrf_modem_dect_phy_tx_params.carrier` field on the transmission parameters.
 
+.. note::
+   The DECT PHY firmware supports band 4 operation on nRF9151 LACA A0 SiP only.
+
 .. important::
    Follow the local regulations when transmitting.
    To see which frequency channels can be used:
-
    * In the European Union, you must refer to table 4.3.2.3-2 of `ETSI EN 301 406-2`_.
    * In the USA, you must refer to `47 CFR Part 15.323`_.
    * In all other territories, you must follow the local regulations.
-
+   When operating on band 4, the carries outside of the [525, 551] range shall not be used as they interfere with other radio devices, including LTE devices.
 
 Transmission length
 -------------------