cfg80211: docbook: add interface combinations documentation

Add the ieee80211_iface_limit and the ieee80211_iface_combination
structures to docbook.  Reformat the examples of combinations
slightly, so it looks a bit better on docbook.

Signed-off-by: Luciano Coelho <luciano.coelho@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>