Fix example description to match option under test
Broaden the path types for setting formatters.

By inversing the check for what a `path`, a broader range of types are
accepted:

  - `String` paths
  - `Pathname` paths
  - `File`
  - Any `IO`
  - `StringIO`
  - Custom "streams"

Do not use `IO === p` for the path check as this does not permit
`StringIO` streams[1]. Use `puts` for the `respond_to` check, instead of
the stream-y `write`, since several internal "streams" only implement
`puts` (`RaiseErrorStream` and `FileStream`).

[1] https://www.ruby-forum.com/topic/127817
Include fix in Changelog.
Merge pull request #1478 from rspec/deprecation-stream-pathname

Support `Pathname` for output streams.