wifi: ieee80211: document two FTM related functions

Add some documentation to ieee80211_is_timing_measurement()
and ieee80211_is_ftm().

Link: https://msgid.link/20240515093852.229aa69e972c.Ifae6762a698e79cd5a49a055fe4c32330e826200@changeid


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