DOC: Correct and expand Series.nonzero docstring
DOC: Correct and expand Series.nonzero docstring

- Mention that a tuple of indices is returned and explain why.
- Instead of numpy.ndarray.nonzero, point to numpy.nonzero, which has
  the full documentation.
- Add example.

nmartensen/pandas,pratapvardhan/pandas,cbertinato/pandas,harisbal/pandas,louispotok/pandas,linebp/pandas,pandas-dev/pandas,cython-testbed/pandas,cython-testbed/pandas,cython-testbed/pandas,zfrenchee/pandas,gfyoung/pandas,linebp/pandas,datapythonista/pandas,cbertinato/pandas,winklerand/pandas,TomAugspurger/pandas,GuessWhoSamFoo/pandas,DGrady/pandas,gfyoung/pandas,harisbal/pandas,amolkahat/pandas,Winand/pandas,jreback/pandas,GuessWhoSamFoo/pandas,harisbal/pandas,dsm054/pandas,TomAugspurger/pandas,dsm054/pandas,gfyoung/pandas,cython-testbed/pandas,MJuddBooth/pandas,pratapvardhan/pandas,winklerand/pandas,toobaz/pandas,louispotok/pandas,toobaz/pandas,pratapvardhan/pandas,amolkahat/pandas,amolkahat/pandas,kdebrab/pandas,jreback/pandas,TomAugspurger/pandas,louispotok/pandas,jmmease/pandas,kdebrab/pandas,MJuddBooth/pandas,jreback/pandas,winklerand/pandas,nmartensen/pandas,cbertinato/pandas,nmartensen/pandas,winklerand/pandas,Winand/pandas,jmmease/pandas,harisbal/pandas,rs2/pandas,nmartensen/pandas,Winand/pandas,GuessWhoSamFoo/pandas,DGrady/pandas,zfrenchee/pandas,toobaz/pandas,toobaz/pandas,linebp/pandas,dsm054/pandas,zfrenchee/pandas,kdebrab/pandas,pandas-dev/pandas,louispotok/pandas,louispotok/pandas,nmartensen/pandas,winklerand/pandas,DGrady/pandas,linebp/pandas,jreback/pandas,datapythonista/pandas,kdebrab/pandas,MJuddBooth/pandas,MJuddBooth/pandas,dsm054/pandas,GuessWhoSamFoo/pandas,linebp/pandas,jorisvandenbossche/pandas,kdebrab/pandas,DGrady/pandas,harisbal/pandas,linebp/pandas,GuessWhoSamFoo/pandas,Winand/pandas,pratapvardhan/pandas,jorisvandenbossche/pandas,amolkahat/pandas,jmmease/pandas,rs2/pandas,jreback/pandas,MJuddBooth/pandas,jmmease/pandas,Winand/pandas,jorisvandenbossche/pandas,DGrady/pandas,pandas-dev/pandas,jmmease/pandas,winklerand/pandas,Winand/pandas,cbertinato/pandas,cython-testbed/pandas,DGrady/pandas,jorisvandenbossche/pandas,amolkahat/pandas,datapythonista/pandas,zfrenchee/pandas,pratapvardhan/pandas,datapythonista/pandas,TomAugspurger/pandas,pandas-dev/pandas,rs2/pandas,zfrenchee/pandas,jmmease/pandas,gfyoung/pandas,cbertinato/pandas,rs2/pandas,dsm054/pandas,gfyoung/pandas,toobaz/pandas,nmartensen/pandas