os: mention fsys modifications during CopyFS

Fixes #70465

Change-Id: I47055df9ca5b1df21a361b0b8eea4c7d157e6403
Reviewed-on: https://go-review.googlesource.com/c/go/+/639156
Commit-Queue: Ian Lance Taylor <iant@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>

Author: seankhliao

src/os/dir.go

@@ -145,6 +145,9 @@ func ReadDir(name string) ([]DirEntry, error) {
 //
 // Symbolic links in dir are followed.
 //
+// New files added to fsys (including if dir is a subdirectory of fsys)
+// while CopyFS is running are not guaranteed to be copied.
+//
 // Copying stops at and returns the first error encountered.
 func CopyFS(dir string, fsys fs.FS) error {
 	return fs.WalkDir(fsys, ".", func(path string, d fs.DirEntry, err error) error {