wifi: ath11k: document HAL_RX_BUF_RBM_SW4_BM

Commit 7636c9a6

 ("wifi: ath11k: Add multi TX ring support for WCN6750")
added HAL_RX_BUF_RBM_SW4_BM to enum hal_rx_buf_return_buf_manager. However,
as flagged by the kernel-doc script, the documentation was not updated:

drivers/net/wireless/ath/ath11k/hal.h:689: warning: Enum value 'HAL_RX_BUF_RBM_SW4_BM' not described in enum 'hal_rx_buf_return_buf_manager'

So update the documentation. No functional changes, compile tested only.

Signed-off-by: Jeff Johnson <quic_jjohnson@quicinc.com>
Signed-off-by: Kalle Valo <quic_kvalo@quicinc.com>
Link: https://msgid.link/20240111-document-hal_rx_buf_rbm_sw4_bm-v1-1-ad277e8ab3cc@quicinc.com