mention tx power in the readme

Author: paul-szczepanek-arm

features/FEATURE_BLE/targets/TARGET_CORDIO/doc/HCIAbstraction.md

@@ -42,3 +42,9 @@ A partial implementation is present in the file
 delivering memory to the stack and a complete reset sequence. However, it does
 not define any initialization for the Bluetooth controller, this part being
 specific to the controller used.
+
+The driver allows to set preferred TX power. This is an optional feature
+(check return code) and the support for it and the extent of control is
+down to the BLE chip. Chips may have different allowed values, although the
+command will pick the closest value no smaller than requested. Please refer
+to the controller code or BLE chip documentation.