Fix DeflateStream.Flush() summary: method is not a no-op when compressing (#12756)

Author: Copilot

xml/System.IO.Compression/DeflateStream.xml

@@ -1102,11 +1102,11 @@ The end write call is invalid.</exception>
       </ReturnValue>
       <Parameters />
       <Docs>
-        <summary>The current implementation of this method has no functionality.</summary>
+        <summary>Flushes the internal buffer to the underlying stream.</summary>
         <remarks>
           <format type="text/markdown"><![CDATA[
 
-Flushes the internal buffer if the compression mode is set to <xref:System.IO.Compression.CompressionMode.Compress>.
+If the compression mode is set to <xref:System.IO.Compression.CompressionMode.Compress>, this method flushes the internal buffer to the underlying stream. If the compression mode is set to <xref:System.IO.Compression.CompressionMode.Decompress>, this method has no effect.
 
  ]]></format>
         </remarks>