Update CharSet.Ansi summary (#2344)

* Update CharSet.Ansi summary

* Add reference to Windows docs

Author: nxtn

xml/System.Runtime.InteropServices/CharSet.xml

@@ -94,7 +94,7 @@
       </ReturnValue>
       <MemberValue>2</MemberValue>
       <Docs>
-        <summary>Marshal strings as multiple-byte character strings.</summary>
+        <summary>Marshal strings as multiple-byte character strings: the system default [Windows (ANSI) code page](/windows/win32/intl/code-pages) on Windows, and UTF-8 on Unix.</summary>
       </Docs>
     </Member>
     <Member MemberName="Auto">
@@ -201,4 +201,4 @@
       </Docs>
     </Member>
   </Members>
-</Type>
+</Type>