DOC: Alligned docstring for ignore_index in append
- [x] closes #xxxx
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

While working on #34932 I found out that `DataFrame.append` and `Series.append` were the only methods with `ignore_index` where the docstring was different. Makes sense - ping on green Thanks @erfannariman 