Enhance FilledButton and Theme Data's documentation to clarify platform-specific visual density behavior (#173695)

- Fix https://github.com/flutter/flutter/issues/155768
- Description: The issue is not a bug but WAI, it's due to visual
density not being the same on each platform by default (see my full
explanation at
[here](https://github.com/flutter/flutter/issues/155768#issuecomment-3178302255)),
see:


f22787193b/packages/flutter/lib/src/material/theme_data.dart (L3250-L3255)

 - This is LOCs where visual density impacts the padding:


a04fb324be/packages/flutter/lib/src/material/button_style_button.dart (L501-L505)



In this PR:
- Add `Visual density effects` section to FilledButton documentation to
avoid confusion for users
- Improve the documentation for
`VisualDensity.defaultDensityForPlatform` by including detailed
information about the specific density for each platform.

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above.
- [x] I updated/added relevant documentation (doc comments with `///`).
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Signed-off-by: huycozy <huy@nevercode.io>

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

@@ -61,6 +61,14 @@ enum _FilledButtonVariant { filled, tonal }
 /// ** See code in examples/api/lib/material/filled_button/filled_button.0.dart **
 /// {@end-tool}
 ///
+/// ## Visual density effects
+///
+/// The button's appearance is affected by the [VisualDensity] from the enclosing
+/// [Theme] or from its [style]. Visual density adjusts the [ButtonStyle.padding]
+/// and [ButtonStyle.minimumSize] to accommodate different UI densities across platforms.
+/// See [VisualDensity] for more details on how it affects component layout and
+/// the platform-specific defaults.
+///
 /// See also:
 ///
 ///  * [ElevatedButton], a filled button whose material elevates when pressed.

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

@@ -3153,6 +3153,10 @@ class _FifoCache<K, V> {
 /// the Material Design specification. It does not affect text sizes, icon
 /// sizes, or padding values.
 ///
+/// The default visual density varies by platform: mobile platforms (Android, iOS,
+/// Fuchsia) use [VisualDensity.standard], while desktop platforms (macOS, Windows,
+/// Linux) use [VisualDensity.compact]. See [defaultDensityForPlatform] for more details.
+///
 /// For example, for buttons, it affects the spacing around the child of the
 /// button. For lists, it affects the distance between baselines of entries in
 /// the list. For chips, it only affects the vertical size, not the horizontal
@@ -3163,6 +3167,7 @@ class _FifoCache<K, V> {
 ///  * [Checkbox]
 ///  * [Chip]
 ///  * [ElevatedButton]
+///  * [FilledButton]
 ///  * [IconButton]
 ///  * [InputDecorator] (which gives density support to [TextField], etc.)
 ///  * [ListTile]
@@ -3240,8 +3245,8 @@ class VisualDensity with Diagnosticable {
 
   /// Returns a [VisualDensity] that is adaptive based on the given [platform].
   ///
-  /// For desktop platforms, this returns [compact], and for other platforms, it
-  /// returns a default-constructed [VisualDensity].
+  /// For mobile platforms (Android, iOS, Fuchsia), this returns [VisualDensity.standard],
+  /// and for desktop platforms (macOS, Windows, Linux), it returns [VisualDensity.compact].
   ///
   /// See also:
   ///