DOC: update the DataFrame.iat[] docstring (#20219)
DOC: update the DataFrame.iat[] docstring (#20219)

* DOC: update the DataFrame.iat[] docstring

* Update based on PR comments

* Update based on PR comments

* Singular not plural

* Update to account for use with Series. Add example using Series.

* Update indexing.py

* PEP8

pratapvardhan/pandas,TomAugspurger/pandas,toobaz/pandas,cython-testbed/pandas,pratapvardhan/pandas,gfyoung/pandas,cython-testbed/pandas,harisbal/pandas,louispotok/pandas,harisbal/pandas,harisbal/pandas,gfyoung/pandas,MJuddBooth/pandas,pratapvardhan/pandas,GuessWhoSamFoo/pandas,harisbal/pandas,datapythonista/pandas,datapythonista/pandas,dsm054/pandas,rs2/pandas,kdebrab/pandas,cython-testbed/pandas,gfyoung/pandas,amolkahat/pandas,pandas-dev/pandas,toobaz/pandas,gfyoung/pandas,jorisvandenbossche/pandas,MJuddBooth/pandas,MJuddBooth/pandas,TomAugspurger/pandas,cbertinato/pandas,cbertinato/pandas,amolkahat/pandas,kdebrab/pandas,jreback/pandas,jorisvandenbossche/pandas,pratapvardhan/pandas,jreback/pandas,MJuddBooth/pandas,TomAugspurger/pandas,amolkahat/pandas,jorisvandenbossche/pandas,louispotok/pandas,rs2/pandas,TomAugspurger/pandas,louispotok/pandas,cbertinato/pandas,gfyoung/pandas,dsm054/pandas,cbertinato/pandas,amolkahat/pandas,cython-testbed/pandas,GuessWhoSamFoo/pandas,MJuddBooth/pandas,datapythonista/pandas,toobaz/pandas,rs2/pandas,GuessWhoSamFoo/pandas,pandas-dev/pandas,rs2/pandas,toobaz/pandas,harisbal/pandas,pandas-dev/pandas,dsm054/pandas,pratapvardhan/pandas,jreback/pandas,jreback/pandas,louispotok/pandas,cython-testbed/pandas,kdebrab/pandas,GuessWhoSamFoo/pandas,jreback/pandas,pandas-dev/pandas,datapythonista/pandas,GuessWhoSamFoo/pandas,dsm054/pandas,dsm054/pandas,toobaz/pandas,cbertinato/pandas,louispotok/pandas,amolkahat/pandas,kdebrab/pandas,jorisvandenbossche/pandas,kdebrab/pandas