DOC: update the DataFrame.at[] docstring (#20290)
DOC: update the DataFrame.at[] docstring (#20290)

* DOC: update the DataFrame.at[] docstring

* Fix line length

* Update based on comments

* Update indexing.py

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