Correct doc for find_cycle() to describe exception when no cycle's found.
Correct doc for find_cycle() to describe exception when no cycle's found.

michaelpacer/networkx,goulu/networkx,cmtm/networkx,jfinkels/networkx,NvanAdrichem/networkx,andnovar/networkx,SanketDG/networkx,Sixshaman/networkx,JamesClough/networkx,OrkoHunter/networkx