Update image docs that Image.file() cashes file (#62301)

packages/flutter/lib/src/painting/image_provider.dart

@@ -837,6 +837,9 @@ abstract class NetworkImage extends ImageProvider<NetworkImage> {
 /// Decodes the given [File] object as an image, associating it with the given
 /// scale.
 ///
+/// The provider does not monitor the file for changes. If you expect the
+/// underlying data to change, you should call the [evict] method.
+///
 /// See also:
 ///
 ///  * [Image.file] for a shorthand of an [Image] widget backed by [FileImage].

packages/flutter/lib/src/widgets/image.dart

@@ -436,6 +436,15 @@ class Image extends StatefulWidget {
   /// will be rendered to the constraints of the layout or [width] and [height]
   /// regardless of these parameters. These parameters are primarily intended
   /// to reduce the memory usage of [ImageCache].
+  ///
+  /// Loading an image from a file creates an in memory copy of the file,
+  /// which is retained in the [ImageCache]. The underlying file is not
+  /// monitored for changes. If it does change, the application should evict
+  /// the entry from the [ImageCache].
+  ///
+  /// See also:
+  ///
+  ///  * [FileImage] provider for evicting the underlying file easily.
   Image.file(
     File file, {
     Key key,