nl80211: docs: add a description for s1g_cap parameter

Changeset df78a0c0 ("nl80211: S1G band and channel definitions")
added a new parameter, but didn't add the corresponding kernel-doc
markup, as repoted when doing "make htmldocs":

	./include/net/cfg80211.h:471: warning: Function parameter or member 's1g_cap' not described in 'ieee80211_supported_band'

Add a documentation for it.

Fixes: df78a0c0 ("nl80211: S1G band and channel definitions")
Signed-off-by: Thomas Pedersen <thomas@adapt-ip.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>

include/net/cfg80211.h

@@ -449,6 +449,7 @@ struct ieee80211_sta_s1g_cap {
  * @n_bitrates: Number of bitrates in @bitrates
  * @ht_cap: HT capabilities in this band
  * @vht_cap: VHT capabilities in this band
+ * @s1g_cap: S1G capabilities in this band
  * @edmg_cap: EDMG capabilities in this band
  * @n_iftype_data: number of iftype data entries
  * @iftype_data: interface type data entries.  Note that the bits in