more doc to pruner.h
more doc to pruner.h

fplll/fplll,fplll/fplll,fplll/fplll,cr-marcstevens/fplll,cr-marcstevens/fplll,fplll/fplll,dstehle/fplll,dstehle/fplll,fplll/fplll,cr-marcstevens/fplll,cr-marcstevens/fplll,dstehle/fplll