Add XML doc comments for StreamExtensions.CreateString and SpanAction

src/Shared/Microsoft.AspNetCore.Razor.Utilities.Shared/StringExtensions.cs

@@ -555,8 +555,47 @@ internal static class StringExtensions
 #endif
     }
 
+    /// <summary>
+    ///  Encapsulates a method that receives a span of objects of type <typeparamref name="T"/>
+    ///  and a state object of type <typeparamref name="TArg"/>.
+    /// </summary>
+    /// <typeparam name="T">
+    ///  The type of the objects in the span.
+    /// </typeparam>
+    /// <typeparam name="TArg">
+    ///  The type of the object that represents the state.
+    /// </typeparam>
+    /// <param name="span">
+    ///  A span of objects of type <typeparamref name="T"/>.
+    /// </param>
+    /// <param name="arg">
+    ///  A state object of type <typeparamref name="TArg"/>.
+    /// </param>
     public delegate void SpanAction<T, in TArg>(Span<T> span, TArg arg);
 
+    /// <summary>
+    ///  Creates a new string with a specific length and initializes it after creation by using the specified callback.
+    /// </summary>
+    /// <typeparam name="TState">
+    ///  The type of the element to pass to <paramref name="action"/>.
+    /// </typeparam>
+    /// <param name="length">
+    ///  The length of the string to create.
+    /// </param>
+    /// <param name="state">
+    ///  The element to pass to <paramref name="action"/>.
+    /// </param>
+    /// <param name="action">
+    ///  A callback to initialize the string
+    /// </param>
+    /// <returns>
+    ///  The created string.
+    /// </returns>
+    /// <remarks>
+    ///  The initial content of the destination span passed to <paramref name="action"/> is undefined.
+    ///  Therefore, it is the delegate's responsibility to ensure that every element of the span is assigned.
+    ///  Otherwise, the resulting string could contain random characters
+    /// </remarks>
     public unsafe static string CreateString<TState>(int length, TState state, SpanAction<char, TState> action)
     {
 #if NET