Update UnmanagedCallersOnlyAttribute.xml

Author: AaronRobinsonMSFT

xml/System.Runtime.InteropServices/UnmanagedCallersOnlyAttribute.xml

@@ -27,9 +27,9 @@
 ## Remarks
 
 Methods marked with this attribute have the following restrictions:
-              * Method must be marked "static".
+              * Method must be marked `static`.
               * Must not be called from managed code.
-              * Must only have <see href="https://docs.microsoft.com/dotnet/framework/interop/blittable-and-non-blittable-types">blittable</see> arguments.
+              * Must only have [blittable](https://docs.microsoft.com/dotnet/framework/interop/blittable-and-non-blittable-types) arguments.
 
           ]]></format>
     </remarks>
@@ -75,7 +75,10 @@ Methods marked with this attribute have the following restrictions:
 ## Remarks
 
 Supplied types must be from the official <xref:System.Runtime.CompilerServices> namespace and be of the form `CallConvXXX`.
-        
+
+The "default platform" calling convention is well defined for all supported platforms except for x86 based platforms. The x86 defaults are as follows:
+   * Windows - [stdcall](https://docs.microsoft.com/cpp/cpp/stdcall)
+   * Non-Windows - [cdecl](https://docs.microsoft.com/cpp/cpp/cdecl)
           ]]></format>
         </remarks>
       </Docs>