Motivation:

NIOFileSystem currently exposes the same API as _NIOFileSystem, which is not API stable. NIOFileSystem
was created in error, and its lack of underscore incorrectly implies API stability. Users who are currently importing NIOFileSystem should ideally move to _NIOFileSystem. However this isn't made clear in the docs:

1. The README talks about the non-underscored NIOFileSystem.
2. There are no hosted API docs NIOFileSystem.
3. The hosted API docs for NIO point to the docs for NIOFileSystem -- results in 404.

Modifications:

• Update the README to refer to _NIOFileSystem.
• Update the hosted API docs for NIO to point to docs for _NIOFileSystem.
• Add hosted API docs for NIOFileSystem with a statement explaining the situation and pointing people to the docs
  for _NIOFileSystem.

Result:

Clearer documentation on the state and relationship of the NIOFileSystem and _NIOFileSystem modules.