docs: fota: Add more details to FOTA module documentation

Add some more detail on the FOTA module operations.

Signed-off-by: Jan Tore Guggedal <jantore.guggedal@nordicsemi.no>

Author: jtguggedal

docs/modules/fota.md

@@ -1,6 +1,27 @@
-# FOTA module
+# FOTA Module
 
-The FOTA (Firmware Over-The-Air) module manages firmware updates for both the application and, if supported, modem firmware. It interacts with nRF Cloud to check for available updates, coordinates the download of new firmware images, and handles the application or activation of the firmware.
+The FOTA (Firmware Over-The-Air) module manages remote firmware updates for both application and modem firmware. It handles all the stages of the update process:
+
+*   Polling nRF Cloud for available updates
+*   Downloading firmware images
+*   Applying updates
+
+The update process begins when the module receives a `FOTA_POLL_REQUEST` message, typically triggered by the main application module. When an update is available, the module automatically initiates the download without requiring additional commands.
+
+The module supports three firmware image types:
+
+*   **Application**: Updates the main application firmware
+*   **Delta Modem**: Incremental modem firmware updates for minor version changes
+*   **Full Modem**: Complete modem firmware replacements
+
+Application and delta modem images are marked ready for their respective bootloaders after download is completed and applied automatically on the next device reboot. When these downloads complete successfully, the module publishes a `FOTA_SUCCESS_REBOOT_NEEDED` message to indicate that a reboot is required to apply the update.
+
+Full modem updates require that the device disconnects from the network before applying the update. For these updates, the module first sends a `FOTA_IMAGE_APPLY_NEEDED` message. The main application must then:
+1. Disconnect from the cellular network
+2. Send a `FOTA_IMAGE_APPLY` message to the FOTA module
+3. Wait for the `FOTA_SUCCESS_REBOOT_NEEDED` message that indicates successful application of the new modem firmware.
+
+All update operations feature error handling with appropriate status messages, allowing the application to recover gracefully from download failures or interruptions.
 
 ## Messages