Add XML documentation for TextWatcher event args

[Copilot]

Android.Text.TextChangedEventArgs and Android.Text.AfterTextChangedEventArgs are public, user-facing types surfaced by TextView text-change events, but they had no IntelliSense documentation. This adds XML docs to the hand-written event args in ITextWatcher.cs without changing behavior or touching the internal implementor.

• Document public event args

  • Added <summary> docs to:
    • AfterTextChangedEventArgs
    • TextChangedEventArgs
  • Clarified which TextView events each type backs.

• Document public members

  • Added constructor <param> docs for:
    • editable
    • text
    • start
    • before
    • after
  • Added property docs for:
    • Editable
    • Text
    • Start
    • BeforeCount
    • AfterCount

• Align with Android callback semantics

  • Documented TextChangedEventArgs in terms of TextWatcher.beforeTextChanged() / onTextChanged().
  • Documented AfterTextChangedEventArgs in terms of TextWatcher.afterTextChanged().
  • Added seealso links to the corresponding Android API docs.

• Scope

  • Leaves TextWatcherImplementor unchanged.
  • No runtime or API shape changes; comments only.

Example of the added documentation pattern:

/// <summary>
/// Provides data for the <see cref="Android.Widget.TextView.AfterTextChanged"/> event, raised after the text has been changed.
/// </summary>
/// <seealso href="https://developer.android.com/reference/android/text/TextWatcher#afterTextChanged(android.text.Editable)">Android documentation for <c>android.text.TextWatcher.afterTextChanged</c></seealso>
public class AfterTextChangedEventArgs : EventArgs {

	/// <summary>
	/// Gets the mutable <see cref="IEditable"/> instance that was changed, or <see langword="null"/>.
	/// </summary>
	public IEditable? Editable {
		get { return editable; }
	}
}