Add a note on watching files on Linux.

sdk/lib/io/file_system_entity.dart

Index: sdk/lib/io/file_system_entity.dart
diff --git a/sdk/lib/io/file_system_entity.dart b/sdk/lib/io/file_system_entity.dart
index 16fb764718e6363fc650ee55401aade71123129c..60fe6658dd41fa3d30fc28528dfc7295dc8873dd 100644
--- a/sdk/lib/io/file_system_entity.dart
+++ b/sdk/lib/io/file_system_entity.dart
@@ -349,6 +349,8 @@ abstract class FileSystemEntity {
    *     supports watching directories. Recursive watching is supported.
    *   * `Linux`: Uses `inotify`. The implementation supports watching both
    *     files and directories. Recursive watching is not supported.
+   *     Note: When watching files directly, delete events might not happen
+   *     as expected.
    *   * `Mac OS`: Uses `FSEvents`. The implementation supports watching both
    *     files and directories. Recursive watching is supported.
    *     Note: events happened slightly before calling [watch], may be part of