TYP: Overload series/ffill and bfill (#41152)
TYP: Overload series/ffill and bfill (#41152)

* TYP: Overload series/ffill and bfill

* Resolved comments

* Keep the return types
rs2/pandas,jorisvandenbossche/pandas,datapythonista/pandas,dsm054/pandas,pandas-dev/pandas,gfyoung/pandas,datapythonista/pandas,pandas-dev/pandas,jorisvandenbossche/pandas,jorisvandenbossche/pandas,rs2/pandas,dsm054/pandas,datapythonista/pandas,pandas-dev/pandas,dsm054/pandas,gfyoung/pandas,datapythonista/pandas,gfyoung/pandas,jorisvandenbossche/pandas,dsm054/pandas,gfyoung/pandas,pandas-dev/pandas,rs2/pandas,gfyoung/pandas,dsm054/pandas,rs2/pandas