Added docstring to nBest() in translator/history.h, as per code review.
Added docstring to nBest() in translator/history.h, as per code review.

emjotde/amunn,emjotde/amunmt,marian-nmt/marian-train,marian-nmt/marian-train,emjotde/Marian,marian-nmt/marian-train,marian-nmt/marian-train,emjotde/amunmt,emjotde/amunn,emjotde/amunmt,emjotde/amunn,emjotde/amunn,emjotde/amunmt,emjotde/Marian,marian-nmt/marian-train