[Docs] update Button.md

[manabu-nakamura]

closes #4385

[manabu-nakamura]

We don't need to change the following description (res/values/styles.xml), but it is difficult to understand:

<style name="ShapeAppearance.App.SmallComponent" parent="ShapeAppearance.Material3.SmallComponent">
    <item name="cornerFamily">cut</item>
    <item name="cornerSize">4dp</item>
</style>

<style name="Theme.App" parent="Theme.Material3.*">
    ...
    <item name="borderlessButtonStyle">@style/Widget.App.Button.TextButton</item>
    <item name="materialButtonOutlinedStyle">@style/Widget.App.Button.OutlinedButton</item>
    <item name="materialButtonStyle">@style/Widget.App.Button</item>
</style>

<style name="Widget.App.Button.TextButton" parent="Widget.Material3.Button.TextButton">
    <item name="materialThemeOverlay">@style/ThemeOverlay.App.Button.TextButton</item>
    <item name="android:textAppearance">@style/TextAppearance.App.Button</item>
    <item name="shapeAppearance">@style/ShapeAppearance.App.SmallComponent</item>
</style>

<style name="Widget.App.Button.OutlinedButton" parent="Widget.Material3.Button.OutlinedButton">
    <item name="materialThemeOverlay">@style/ThemeOverlay.App.Button.TextButton</item>
    <item name="android:textAppearance">@style/TextAppearance.App.Button</item>
    <item name="shapeAppearance">@style/ShapeAppearance.App.SmallComponent</item>
</style>

<style name="Widget.App.Button" parent="Widget.Material3.Button">
    <item name="materialThemeOverlay">@style/ThemeOverlay.App.Button</item>
    <item name="android:textAppearance">@style/TextAppearance.App.Button</item>
    <item name="shapeAppearance">@style/ShapeAppearance.App.SmallComponent</item>
</style>

<style name="ThemeOverlay.App.Button.TextButton" parent="ThemeOverlay.Material3.Button.TextButton">
    <item name="colorOnContainer">@color/shrine_pink_900</item>
</style>

<style name="ThemeOverlay.App.Button" parent="ThemeOverlay.Material3.Button">
    <item name="colorContainer">@color/shrine_pink_100</item>
    <item name="colorOnContainer">@color/shrine_pink_900</item>
</style>

[manabu-nakamura]

Is it better than https://github.com/material-components/material-components-android/pull/4445/files?

**Shape**        | `app:shapeAppearance` | `setShapeAppearanceModel`<br/>`getShapeAppearanceModel`                      | `50% rounded`

(b184df6)