Document that StreamWriter.Dispose flushes

This is an important detail. It tells you that `Dispose` is a blocking call. It also tells you that it can throw I/O exceptions. And of course that you don't have to call `Flush` or `FlushAsync` yourself first.

Author: breyed

xml/System.IO/StreamWriter.xml

@@ -946,7 +946,7 @@
       <Docs>
         <param name="disposing">
           <see langword="true" /> to release both managed and unmanaged resources; <see langword="false" /> to release only unmanaged resources.</param>
-        <summary>Releases the unmanaged resources used by the <see cref="T:System.IO.StreamWriter" /> and optionally releases the managed resources.</summary>
+        <summary>Causes any buffered data to be written to the underlying stream, releases the unmanaged resources used by the <see cref="T:System.IO.StreamWriter" />, and optionally the managed resources.</summary>
         <remarks>
           <format type="text/markdown"><![CDATA[  
   
@@ -2175,4 +2175,4 @@ For a list of common I/O tasks, see [Common I/O Tasks](~/docs/standard/io/common
       </Docs>
     </Member>
   </Members>
-</Type>
+</Type>