ath10k: Document alloc_frag_desc_for_data_pkt config option.

This will help anyone trying to use the ack-rssi reporting
feature with the host-specified TX-rate option in 10.4 firmware.

Signed-off-by: Ben Greear <greearb@candelatech.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>