cfg80211: add missing kernel-doc for S1G band capabilities

Add missing kernel-doc for the S1G band capabilities in the
per band data.

Link: https://lore.kernel.org/r/20200918131921.08c893cd73a1.Id71583c37baca8a9a3329426e02b66d9ab65ac03@changeid


Signed-off-by: Johannes Berg <johannes.berg@intel.com>