Mention SelectionArea in SelectableText docs (flutter#143784)

Most users should be using SelectionArea over SelectableText, so this makes that more clear/discoverable in the docs.

Author: justinmc

packages/flutter/lib/src/material/selectable_text.dart

@@ -151,6 +151,9 @@ class _SelectableTextSelectionGestureDetectorBuilder extends TextSelectionGestur
 
 /// A run of selectable text with a single style.
 ///
+/// Consider using [SelectionArea] or [SelectableRegion] instead, which enable
+/// selection on a widget subtree, including but not limited to [Text] widgets.
+///
 /// The [SelectableText] widget displays a string of text with a single style.
 /// The string might break across multiple lines or might all be displayed on
 /// the same line depending on the layout constraints.
@@ -214,6 +217,8 @@ class _SelectableTextSelectionGestureDetectorBuilder extends TextSelectionGestur
 ///
 ///  * [Text], which is the non selectable version of this widget.
 ///  * [TextField], which is the editable version of this widget.
+///  * [SelectionArea], which enables the selection of multiple [Text] widgets
+///    and of other widgets.
 class SelectableText extends StatefulWidget {
   /// Creates a selectable text widget.
   ///

packages/flutter/lib/src/material/selection_area.dart

@@ -35,6 +35,7 @@ import 'theme.dart';
 /// See also:
 ///
 ///  * [SelectableRegion], which provides an overview of the selection system.
+///  * [SelectableText], which enables selection on a single run of text.
 class SelectionArea extends StatefulWidget {
   /// Creates a [SelectionArea].
   ///

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

@@ -195,6 +195,7 @@ const double _kSelectableVerticalComparingThreshold = 3.0;
 ///
 ///  * [SelectionArea], which creates a [SelectableRegion] with
 ///    platform-adaptive selection controls.
+///  * [SelectableText], which enables selection on a single run of text.
 ///  * [SelectionHandler], which contains APIs to handle selection events from the
 ///    [SelectableRegion].
 ///  * [Selectable], which provides API to participate in the selection system.