Fixes to functorcollections doc and add MCParticle.h to ThOr functor headers for truth matching

Includes some fixes to functorcollections doc. It also fixes MC association functor F.MAP_INPUT(F.P, MCAssocTable) which crashes when JIT compiling, since there is no MCParticle.h in the header for any functors involved, fixed by adding MCParticle.h to every functor that runs over LHCb::Particle::Range.