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>

src/io/fs/fs.go

@@ -120,6 +120,7 @@ type ReadDirFile interface {
 	// In this case, if ReadDir returns an empty slice, it will return
 	// a non-nil error explaining why.
 	// At the end of a directory, the error is io.EOF.
+	// (ReadDir must return io.EOF itself, not an error wrapping io.EOF.)
 	//
 	// If n <= 0, ReadDir returns all the DirEntry values from the directory
 	// in a single slice. In this case, if ReadDir succeeds (reads all the way