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>