Added missing XML comments

Author: Sergio0694

Microsoft.Toolkit.HighPerformance/Streams/Sources/Interfaces/ISpanOwner.cs

@@ -6,10 +6,19 @@
 
 namespace Microsoft.Toolkit.HighPerformance.Streams
 {
+    /// <summary>
+    /// An interface for types acting as sources for <see cref="Span{T}"/> instances.
+    /// </summary>
     internal interface ISpanOwner
     {
+        /// <summary>
+        /// Gets the length of the underlying memory area.
+        /// </summary>
         int Length { get; }
 
+        /// <summary>
+        /// Gets a <see cref="Span{T}"/> instance wrapping the underlying memory area.
+        /// </summary>
         Span<byte> Span { get; }
     }
 }