Update obsolete message on InplaceStringBuilder

src/libraries/Microsoft.Extensions.Primitives/ref/Microsoft.Extensions.Primitives.cs

@@ -36,7 +36,7 @@ public partial interface IChangeToken
         bool HasChanged { get; }
         System.IDisposable RegisterChangeCallback(System.Action<object?> callback, object? state);
     }
-    [System.ObsoleteAttribute("This type is obsolete and will be removed in a future version.")]
+    [System.ObsoleteAttribute("This method is retained only for compatibility. The recommended alternative is string.Create<TState> (int length, TState state, System.Buffers.SpanAction<char,TState> action).")]
     public partial struct InplaceStringBuilder
     {
         private object _dummy;

src/libraries/Microsoft.Extensions.Primitives/src/InplaceStringBuilder.cs

@@ -8,7 +8,7 @@
 namespace Microsoft.Extensions.Primitives
 {
     [DebuggerDisplay("Value = {_value}")]
-    [Obsolete("This type is obsolete and will be removed in a future version.")]
+    [Obsolete("This method is retained only for compatibility. The recommended alternative is string.Create<TState> (int length, TState state, System.Buffers.SpanAction<char,TState> action).")]
     public struct InplaceStringBuilder
     {
         private int _offset;