fs: clarify documentation for ReadDir method

The fs.ReadDir method behaves the same way as
os.ReadDir, in that when n <= 0, ReadDir returns
all DirEntry values remaining in the dictionary.

Update the comment to reflect that only remaining
DirEntry values are returned (not all entries),
for subsequent calls.

Fixes #69301

Change-Id: I41ef7ef1c8e3fe7d64586f5297512697dc60dd40
Reviewed-on: https://go-review.googlesource.com/c/go/+/663215
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Sean Liao <sean@liao.dev>
Auto-Submit: Sean Liao <sean@liao.dev>
Reviewed-by: Jorropo <jorropo.pgm@gmail.com>
Reviewed-by: David Chase <drchase@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

Author: 13mtfb

src/io/fs/fs.go

@@ -129,7 +129,7 @@ type ReadDirFile interface {
 	// 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
+	// If n <= 0, ReadDir returns all remaining DirEntry values from the directory
 	// in a single slice. In this case, if ReadDir succeeds (reads all the way
 	// to the end of the directory), it returns the slice and a nil error.
 	// If it encounters an error before the end of the directory,