Revise tooltip theme docs, including more cross-references (#137316)

Much of the new wording here is borrowed from [ChipTheme], [SliderTheme], or [RadioThemeData], which I think are pretty good. I believe a lot of other theme classes have similar wording too. I've also made some tweaks of my own, notably the references to [MaterialApp.theme].

This started from a desire to have clearer cross-references pointing at what to do with a FooThemeData to make it take effect:
  https://github.com/flutter/flutter/pull/135879#discussion_r1355851481
but then as I started writing I kept finding more and more small things I wanted to adjust, including a couple of bits that were extraneous or obsolete.

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

@@ -12,20 +12,23 @@ import 'theme.dart';
 // Examples can assume:
 // late BuildContext context;
 
-/// Defines the visual properties of [Tooltip] widgets.
+/// Defines the visual properties of [Tooltip] widgets, a tooltip theme.
 ///
-/// Used by [TooltipTheme] to control the visual properties of tooltips in a
-/// widget subtree.
+/// Each property of [TooltipThemeData] corresponds to a property of [Tooltip],
+/// and describes the value to use when the [Tooltip] property is
+/// not given an explicit non-null value.
 ///
-/// To obtain this configuration, use [TooltipTheme.of] to access the closest
-/// ancestor [TooltipTheme] of the current [BuildContext].
+/// Use this class to configure a [TooltipTheme] widget, or to set the
+/// [ThemeData.tooltipTheme] for a [Theme] widget or [MaterialApp.theme].
+///
+/// To obtain the current ambient tooltip theme, use [TooltipTheme.of].
 ///
 /// See also:
 ///
-///  * [TooltipTheme], an [InheritedWidget] that propagates the theme down its
-///    subtree.
-///  * [TooltipThemeData], which describes the actual configuration of a
-///    tooltip theme.
+///  * [TooltipTheme], a widget which overrides the tooltip theme for a subtree.
+///  * [ThemeData.tooltipTheme], which specifies a tooltip theme as part of
+///    an overall theme.
+///  * [MaterialApp.theme], which specifies a theme for the whole application.
 @immutable
 class TooltipThemeData with Diagnosticable {
   /// Creates the set of properties used to configure [Tooltip]s.
@@ -237,11 +240,13 @@ class TooltipThemeData with Diagnosticable {
   }
 }
 
-/// An inherited widget that defines the configuration for
-/// [Tooltip]s in this widget's subtree.
+/// Applies a tooltip theme to descendant [Tooltip] widgets.
+///
+/// A tooltip theme describes the values to use for [Tooltip] properties
+/// that are not given an explicit non-null value.
 ///
-/// Values specified here are used for [Tooltip] properties that are not
-/// given an explicit non-null value.
+/// Descendant widgets obtain the ambient tooltip theme, a [TooltipThemeData],
+/// using [TooltipTheme.of].
 ///
 /// {@tool snippet}
 ///
@@ -270,6 +275,8 @@ class TooltipThemeData with Diagnosticable {
 ///
 /// See also:
 ///
+///  * [TooltipThemeData], which describes the actual configuration of a
+///    tooltip theme.
 ///  * [TooltipVisibility], which can be used to visually disable descendant [Tooltip]s.
 class TooltipTheme extends InheritedTheme {
   /// Creates a tooltip theme that controls the configurations for
@@ -283,9 +290,13 @@ class TooltipTheme extends InheritedTheme {
   /// The properties for descendant [Tooltip] widgets.
   final TooltipThemeData data;
 
-  /// Returns the [data] from the closest [TooltipTheme] ancestor. If there is
-  /// no ancestor, it returns [ThemeData.tooltipTheme]. Applications can assume
-  /// that the returned value will not be null.
+  /// Returns the ambient tooltip theme.
+  ///
+  /// The result comes from the closest [TooltipTheme] ancestor if any,
+  /// and otherwise from [Theme.of] and [ThemeData.tooltipTheme].
+  ///
+  /// When a widget uses this method, it is automatically rebuilt if the
+  /// tooltip theme later changes, so that the changes can be applied.
   ///
   /// Typical usage is as follows:
   ///