DOC: Indicate that to_csv and from_csv methods can handle string file pa...
...th and file-like objects for io.

It also seems a good idea to align the argument names: _path_, _path_or_buffer_, _filepath_or_buffer_ are all three used among `to_csv()`, `from_csv()`, `read_table()` to indicate the same. I did not touch those since that is an api change.
