docs/examples/sx127x_demo: Expand SX127X demo documentation

Add default configuration, short-range FSK testing tips,
and troubleshooting guidance for the SX127X demo example.

Signed-off-by: Arjav Patel <arjav1528@gmail.com>

Documentation/applications/examples/sx127x_demo/index.rst

@@ -3,3 +3,68 @@
 ============================
 
 This example demonstrates the use of the ``SX127X`` radio.
+
+Overview
+========
+
+The ``sx127x_demo`` application exercises the `SX127X` low-power wide-area
+network (LPWAN) radio driver in both transmit and receive modes.  It is
+typically enabled via a board defconfig together with
+``CONFIG_LPWAN_SX127X`` and the appropriate SPI and GPIO wiring for the
+target module.
+
+Default configuration
+=====================
+
+Unless explicitly overridden in the board or application configuration,
+the SX127X driver uses the following relevant defaults:
+
+* RF frequency: ``CONFIG_LPWAN_SX127X_RFFREQ_DEFAULT`` (board-specific,
+  for example 433 MHz, 868 MHz or 915 MHz).
+* TX power: ``CONFIG_LPWAN_SX127X_TXPOWER_DEFAULT``.
+  The global default is **17 dBm**, which provides a stronger and more
+  reliable link than the previous 14 dBm default.
+* CRC: ``CONFIG_LPWAN_SX127X_CRCON``.
+  The global default is **1** (CRC enabled), so payloads are protected
+  by a checksum and corrupted frames are rejected.
+
+Individual boards may override these values in their ``defconfig`` files.
+For example, some boards tested with integrated SX1276 modules may set
+``CONFIG_LPWAN_SX127X_TXPOWER_DEFAULT=20`` and adjust
+``CONFIG_LPWAN_SX127X_RFFREQ_DEFAULT`` to match the module’s band.
+
+Short-range FSK testing
+=======================
+
+When running short-range FSK tests (for example, two modules placed
+around one meter apart on a lab bench), the following settings are
+recommended:
+
+* Keep ``CONFIG_LPWAN_SX127X_TXPOWER_DEFAULT`` at **17 dBm** or higher
+  (some modules work well at 20 dBm) to avoid marginal RSSI values that
+  can lead to intermittent reception.
+* Ensure ``CONFIG_LPWAN_SX127X_CRCON=1`` on both transmitter and
+  receiver so corrupted frames are discarded instead of being printed
+  with incorrect data.
+
+These defaults are based on empirical testing reported by users and are
+intended to provide robust communication without requiring additional
+per-board tuning.
+
+Troubleshooting
+===============
+
+If the receiver occasionally shows no data, truncated data, or payloads
+that do not match what was transmitted, check the following:
+
+* Verify that the RF frequency and modulation settings are identical on
+  both sides.
+* Inspect the reported RSSI and SNR values; very low RSSI at short
+  distance can indicate insufficient TX power or antenna issues.
+* Confirm that CRC is enabled and that both peers are configured
+  consistently (either both with CRC enabled or both with it disabled,
+  though enabling CRC is recommended).
+
+Adjusting TX power or antenna placement and ensuring consistent CRC
+settings on the link typically resolves data integrity problems seen in
+simple FSK tests.