DOC: Update doc description for day_opt in offsets (#24602)
DOC: Update doc description for day_opt in offsets (#24602)

Unlike shift_month, all other descriptions do not
mention support of "None" as a value, since it
is not supported in the other functions.

Follow-up to gh-24585.
jreback/pandas,GuessWhoSamFoo/pandas,datapythonista/pandas,TomAugspurger/pandas,gfyoung/pandas,toobaz/pandas,gfyoung/pandas,dsm054/pandas,pandas-dev/pandas,TomAugspurger/pandas,MJuddBooth/pandas,pandas-dev/pandas,toobaz/pandas,MJuddBooth/pandas,cbertinato/pandas,toobaz/pandas,rs2/pandas,GuessWhoSamFoo/pandas,datapythonista/pandas,datapythonista/pandas,jorisvandenbossche/pandas,TomAugspurger/pandas,dsm054/pandas,GuessWhoSamFoo/pandas,GuessWhoSamFoo/pandas,MJuddBooth/pandas,pandas-dev/pandas,rs2/pandas,toobaz/pandas,MJuddBooth/pandas,cbertinato/pandas,TomAugspurger/pandas,jorisvandenbossche/pandas,dsm054/pandas,jreback/pandas,rs2/pandas,cbertinato/pandas,datapythonista/pandas,GuessWhoSamFoo/pandas,jreback/pandas,toobaz/pandas,jorisvandenbossche/pandas,gfyoung/pandas,gfyoung/pandas,jreback/pandas,jreback/pandas,dsm054/pandas,MJuddBooth/pandas,rs2/pandas,dsm054/pandas,pandas-dev/pandas,cbertinato/pandas,jorisvandenbossche/pandas,gfyoung/pandas,cbertinato/pandas