docs: Bluetooth: HCI_VS: Updated docs on VS HCI commands

This commit complements documentation of Zephyr HCI VS
commands. In particular, it documents the newly introduced
commands

- BT_HCI_OP_VS_WRITE_TX_POWER_LEVEL
- BT_HCI_OP_VS_READ_TX_POWER_LEVEL

These provide HCI-level control over the Tx power of the
BLE radio on a per role/connection basis.

The functionality is enabled upon the Kconfig advanced configuration
triggered by

- BT_CTLR_TX_PWR_DYNAMIC_CONTROL

depending on the enablement of Zephyr HCI vendor-specific command
extensions.

Signed-off-by: Andrei Stoica <stoica.razvan.andrei@gmail.com>