doc: Refer test commands section of BLE Core Spec in DTM sample

Refer actual encoding of the test commands and events section
of BLE Core Spec in Direct test mode sample.

Signed-off-by: divya pillai <[email protected]>

Author: divipillai

samples/bluetooth/direct_test_mode/README.rst

@@ -8,6 +8,8 @@ Bluetooth: Direct Test Mode
    :depth: 2
 
 This sample enables the Direct Test Mode functions described in `Bluetooth® Core Specification <Bluetooth Core Specification_>`_ (Vol. 6, Part F).
+The actual encoding of the test commands and events are described in sections 3.3 and 3.4, respectively, of Vol. 6, Part F of this specification document.
+The `Vendor-specific packet payload`_ section describes some vendor-specific commands that comply with the core specification.
 
 Requirements
 ************