DOC: clean-up 0.19.0 whatsnew file (#14176)
DOC: clean-up 0.19.0 whatsnew file (#14176)

* DOC: clean-up 0.19.0 whatsnew file
* further clean-up
* Update highlights
* consistent use of behaviour/behavior
* s/favour/favor

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