Updated cWB definition to point out the meaning of BBH and IMBH

The Search is reported in a GCN Notice (labeled as "Search_type") and the only reason 'IMBH' wasn't shown in the 114f notices, https://gcn.gsfc.nasa.gov/notices_l/S200114f.lvc , was because GCN had not been configured to expect 'IMBH' while is configurated to report "BBH" for Burst search. The fix in the documentation will make clear what "BBH" and ÏMBH" refer to: "Tuned versions for binary black holes (BBH, IMBH) choose the time-frequency patterns with frequency increasing in time"