Small fixes to Image docs: NNBD, and add a cross-reference (#151938)

In particular, without this cross-reference it's easy for a reader
looking at [Image.frameBuilder] to think that the meanings of these
parameters just aren't properly documented.  (I had that thought for
a minute, before thinking *surely* they're documented, and then
trying the link to the typedef.)

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

@@ -152,8 +152,8 @@ Future<void> precacheImage(
 /// Signature used by [Image.frameBuilder] to control the widget that will be
 /// used when an [Image] is built.
 ///
-/// The `child` argument contains the default image widget and is guaranteed to
-/// be non-null. Typically, this builder will wrap the `child` widget in some
+/// The `child` argument contains the default image widget.
+/// Typically, this builder will wrap the `child` widget in some
 /// way and return the wrapped widget. If this builder returns `child` directly,
 /// it will yield the same result as if [Image.frameBuilder] was null.
 ///
@@ -174,8 +174,6 @@ Future<void> precacheImage(
 /// frames. In other words, if the first image frame was available immediately,
 /// then this argument will be true for all image frames.
 ///
-/// This builder must not return null.
-///
 /// See also:
 ///
 ///  * [Image.frameBuilder], which makes use of this signature in the [Image]
@@ -212,8 +210,6 @@ typedef ImageFrameBuilder = Widget Function(
 /// In such cases, the `child` parameter will represent the current
 /// fully-loaded image frame.
 ///
-/// This builder must not return null.
-///
 /// See also:
 ///
 ///  * [Image.loadingBuilder], which makes use of this signature in the [Image]
@@ -712,6 +708,9 @@ class Image extends StatefulWidget {
   /// add effects to the image (such as fading the image in when it becomes
   /// available) or to display a placeholder widget while the image is loading.
   ///
+  /// For more information on how to interpret the arguments that are passed to
+  /// this builder, see the documentation on [ImageFrameBuilder].
+  ///
   /// To have finer-grained control over the way that an image's loading
   /// progress is communicated to the user, see [loadingBuilder].
   ///