DOC: update DataFrame.to_feather docstring
- [x] closes #29532
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`

The issue was that `DataFrame.to_feather` had an `fname` argument which in fact could be either a filename or a buffer. Most of the work to close #29532 was done in #30338 where `fname` argument was deprecated in favor of `path`. Here I update the docstring to let users know that they can use a buffer. 
@simonjayhawkins  LGTM thanks @arw2019 