io/fs: document ReadDirFile io.EOF implementation requirements more precisely

[mknyszek]

Splintering out from a discussion in #47062, it's probably worth pointing out that like an io.Reader, an fs.ReadDirFile's ReadDir should return only an io.EOF, and not anything wrapping it. Code in testing/fstest already depends on this behavior and assumes it for a correct implementation.

Alternatively, because this API is relatively new, it might still be possible to relax it to allow for wrapping. However, since it's already been almost one full release and io.EOF is already used a sentinel in other APIs, it doesn't seem worth it to break the pattern.

[gopherbot]

Change https://golang.org/cl/333149 mentions this issue: io/fs: document requirement that ReadDir return an unwrapped io.EOF

[neild]

The fs.ReadDirFile interface is new, but "*os/File".ReadDir is old and returns an io.EOF sentinel without wrapping. Keeping the requirement that io.EOF be used without wrapping seems simplest.