Fix GZipStream.Flush summary and remarks (#3789)

Author: carlossanlop

xml/System.IO.Compression/GZipStream.xml

@@ -980,16 +980,17 @@ Calling `DisposeAsync` allows the resources used by the <xref:System.IO.Compress
       </ReturnValue>
       <Parameters />
       <Docs>
-        <summary>The current implementation of this method has no functionality.</summary>
+        <summary>Flushes the internal buffers.</summary>
         <remarks>
-          <format type="text/markdown"><![CDATA[  
-  
-## Remarks  
- The current implementation of this method does not flush the internal buffer. The internal buffer is flushed when the object is disposed.  
-  
- ]]></format>
+          <format type="text/markdown"><![CDATA[
+
+## Remarks
+
+This method flushes only if the current compression mode is <xref:System.IO.Compression.CompressionMode.Compress> and the underlying stream still has some input left to write.
+
+          ]]></format>
         </remarks>
-        <exception cref="T:System.ObjectDisposedException">The stream is closed.</exception>
+        <exception cref="T:System.ObjectDisposedException">The underlying stream is closed.</exception>
       </Docs>
     </Member>
     <Member MemberName="FlushAsync">