Add warning about fromSignal flag in MCParticle

fromSignal is only valid for signal decays produced by EvtGen, see this reply on lhcb-simulation by @philten.

This has come up before, and just today @dzuliani ran into this problem.
So I figured I'd add this warning to the header in the hope of maybe having fewer people run into this annoying problem.

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• 5b9b3ec0 - Update MCParticle.h

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mentioned in commit 3374eef9

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mentioned in commit 9c5bea95

mentioned in merge request !2807 (merged)