Adding documentation for flatten
Adding documentation for flatten

paultag/hy,farhaven/hy,Tritlo/hy,larme/hy,larme/hy,timmartin/hy,tianon/hy,zackmdavis/hy,Foxboron/hy,farhaven/hy,algernon/hy,kirbyfan64/hy,michel-slm/hy,mtmiller/hy,aisk/hy,ALSchwalm/hy,aisk/hy,tianon/hy,aisk/hy,kirbyfan64/hy,farhaven/hy,Foxboron/hy,jakirkham/hy,freezas/hy,tianon/hy,tuturto/hy,kartikm/hy,adamfeuer/hy,larme/hy,gilch/hy