[Foundation] Fix nullability in NSDirectoryEnumerator.

This is file 4 of 47 files with nullability disabled in Foundation.

Also (by Copilot):

* Improve XML documentation and nullability in NSDirectoryEnumerator.
* Remove placeholder 'To be added.' comments and replace with proper documentation.
* Remove unnecessary whitespace after triple-slash in XML comments.
* Add proper <see cref> attributes for type references.
* Add comprehensive documentation for all IEnumerator implementations.
* Document exception conditions for Reset and Current properties.

Contributes towards #17285.

Author: rolfbjarne

src/Foundation/NSDirectoryEnumerator.cs

@@ -8,50 +8,50 @@
 using System.Collections.Generic;
 using System.Collections;
 
-// Disable until we get around to enable + fix any issues.
-#nullable disable
+#nullable enable
 
 namespace Foundation {
-	public partial class NSDirectoryEnumerator : IEnumerator<NSString>, IEnumerator<string>, IEnumerator {
-		NSObject current;
+	public partial class NSDirectoryEnumerator : IEnumerator<NSString?>, IEnumerator<string?>, IEnumerator {
+		NSObject? current;
 
-		/// <summary>To be added.</summary>
-		///         <returns>To be added.</returns>
-		///         <remarks>To be added.</remarks>
+		/// <summary>Advances the enumerator to the next element of the collection.</summary>
+		/// <returns><see langword="true"/> if the enumerator was successfully advanced to the next element; <see langword="false"/> if the enumerator has passed the end of the collection.</returns>
 		bool IEnumerator.MoveNext ()
 		{
 			current = NextObject ();
 			return current is not null;
 		}
 
-		/// <summary>To be added.</summary>
-		///         <remarks>To be added.</remarks>
+		/// <summary>Sets the enumerator to its initial position, which is before the first element in the collection.</summary>
+		/// <exception cref="InvalidOperationException">This operation is not supported for this enumerator.</exception>
 		void IEnumerator.Reset ()
 		{
 			throw new InvalidOperationException ();
 		}
 
-		/// <summary>Gets the current element.</summary>
-		/// <value>The current element.</value>
-		string IEnumerator<string>.Current {
+		/// <summary>Gets the current element as a <see cref="string"/>.</summary>
+		/// <value>The current element as a <see cref="string"/>, or <see langword="null"/> if the enumerator is positioned before the first element or after the last element.</value>
+		string? IEnumerator<string?>.Current {
 			get {
-				return current.ToString ();
+				return current?.ToString ();
 			}
 		}
 
-		/// <summary>Gets the current element.</summary>
-		/// <value>The current element.</value>
-		NSString IEnumerator<NSString>.Current {
+		/// <summary>Gets the current element as an <see cref="NSString"/>.</summary>
+		/// <value>The current element as an <see cref="NSString"/>, or <see langword="null"/> if the enumerator is positioned before the first element or after the last element, or if the current element is not an <see cref="NSString"/>.</value>
+		NSString? IEnumerator<NSString?>.Current {
 			get {
 				return current as NSString;
 			}
 		}
 
-		/// <summary>Gets the current element.</summary>
-		///         <value>The current element.</value>
-		///         <remarks>To be added.</remarks>
+		/// <summary>Gets the current element in the collection.</summary>
+		/// <value>The current element in the collection.</value>
+		/// <exception cref="InvalidOperationException">The enumerator is positioned before the first element of the collection or after the last element.</exception>
 		object IEnumerator.Current {
 			get {
+				if (current is null)
+					throw new InvalidOperationException ();
 				return current;
 			}
 		}