Add IEffectControlProvider docs

[jknaudt21]

Description of Change

Added the missing docs for the IEffectControlProvider interface

---

Part of documentation effort, related to #3960

[jknaudt21]

I was not entirely clear whether this class is meant for internal use only or open to the public. The implementation inside Element caused these doubts:

maui/src/Controls/src/Core/Element/Element.cs

Lines 387 to 411 in e109469

	/// <summary>For internal use by .NET MAUI.</summary>
	[EditorBrowsable(EditorBrowsableState.Never)]
	public IEffectControlProvider EffectControlProvider
	{
	get { return _effectControlProvider; }
	set
	{
	if (_effectControlProvider == value)
	return;
	if (_effectControlProvider != null && _effects != null)
	{
	foreach (Effect effect in _effects)
	effect?.SendDetached();
	}
	_effectControlProvider = value;
	if (_effectControlProvider != null && _effects != null)
	{
	foreach (Effect effect in _effects)
	{
	if (effect != null)
	AttachEffect(effect);
	}
	}
	}
	}

@jfversluis thoughts?

[jfversluis]

Yeah I think that's fine. The [EditorBrowsable(EditorBrowsableState.Never)] indeed means internal only although there is nothing stopping people from actually using it, it just hides it in IntelliSense. But that has historical reasons. But those are indeed internal, however they should still be documented to achieve more coverage!