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>

include/net/cfg80211.h

@@ -465,6 +465,7 @@ struct ieee80211_sta_s1g_cap {
  * @ht_cap: HT capabilities in this band
  * @vht_cap: VHT capabilities in this band
  * @edmg_cap: EDMG capabilities in this band
+ * @s1g_cap: S1G capabilities in this band (S1B band only, of course)
  * @n_iftype_data: number of iftype data entries
  * @iftype_data: interface type data entries.  Note that the bits in
  *	@types_mask inside this structure cannot overlap (i.e. only