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>