Added note about caching to DllImportResolver (#11864)

Author: gfoidl

xml/System.Runtime.InteropServices/DllImportResolver.xml

@@ -39,6 +39,8 @@
 
 The runtime does not do any lifetime management around the handle returned by a <xref:System.Runtime.InteropServices.DllImportResolver>. It is left to the implementation and consuming code to keep the library loaded for as long as necessary and free it if/when desired. 
 
+The resolver delegate is called everytime when a PInvoke call is done. It is fine to cache the handle, as long as the consuming code does not call <xref:System.Runtime.InteropServices.NativeLibrary.Free>.
+
       ]]></format>
     </remarks>
   </Docs>