wil6210: Tx/Rx descriptors documentation

Sync documentation for the Tx/Rx descriptors with the
firmware/hardware documentation.

Change-Id: I4b9bcfac3c50f47c584a08e1705206016cf94341
Signed-off-by: Vladimir Kondratiev <qca_vkondrat@qca.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Git-commit: 8d3b2f033f8ceb30db60fd23ffbf9525ed2456f4
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/linville/wireless-testing.git


Signed-off-by: Hamad Kadmany <hkadmany@codeaurora.org>