Expand DefaultBufferSize remarks (#3146)

Author: tdykstra

xml/System.Text.Json/JsonSerializerOptions.xml

@@ -118,7 +118,12 @@ Once serialization or deserialization occurs, the list cannot be modified.
 
 ## Remarks
 
-The default buffer size, in bytes, is 16384.
+The default buffer size, in bytes, is 16,384.
+For most workloads, the default size is a reasonable amount of JSON to buffer while reading from a stream or writing to a stream.
+That is, it performs well without creating objects on the Large Object Heap for the Garbage Collector (GC) to track.
+In non-streaming scenarios, increasing the default buffer size could improve performance for large JSON strings or UTF-8 byte arrays.
+We recommend that you leave this value unchanged unless changing it makes an observable difference in performance.
+
 
           ]]></format>
         </remarks>