remove remarks about ref struct (#2863)

Author: mairaw

xml/System.Text.Json/Utf8JsonWriter.xml

@@ -32,8 +32,6 @@
 
 A method that attempts to write invalid JSON when validation is enabled throws a <xref:System.InvalidOperationException> with a context-specific error message.
 
-Since this type is a ref struct, it does not directly support async. However, it does provide support for reentrancy to write partial data and to continue writing in chunks.
-
 To be able to format the output with indentation and white space OR to skip validation, create an instance of <xref:System.Text.Json.JsonWriterOptions> and pass it in to the writer.
 
  ]]></format>