io/fs: document requirement that ReadDir return an unwrapped io.EOF

io/fs: document requirement that ReadDir return an unwrapped io.EOF

This requirement ensures that ReadDir implementations are as compatible
as possible with "*os.File".ReadDir.

The testing/fstest package already tests for equality to io.EOF.

Updates #47062.
Fixes #47086.

Change-Id: I54f911a34e507a3db0abc4da55a19b7a50b35041
Reviewed-on: https://go-review.googlesource.com/c/go/+/333149
Reviewed-by: Michael Knyszek <mknyszek@google.com>