rtlwifi: btcoex: 23b 2ant: add comments to describe how duration adjusted

Since btcoex uses static variables to store the cumulative information
on the wifi status, some tricks are used that are a bit obscure. We add
some comments about the criteria we use to adjust wifi duration.

Signed-off-by: Yan-Hsuan Chuang <yhchuang@realtek.com>
Signed-off-by: Larry Finger <Larry.Finger@lwfinger.net>
Cc: Pkshih <pkshih@realtek.com>
Cc: Birming Chiu <birming@realtek.com>
Cc: Shaofu <shaofu@realtek.com>
Cc: Steven Ting <steventing@realtek.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>