Update UnmanagedCallersOnlyAttribute.xml

Author: AaronRobinsonMSFT

xml/System.Runtime.InteropServices/UnmanagedCallersOnlyAttribute.xml

@@ -76,7 +76,7 @@ Methods marked with this attribute have the following restrictions:
 
 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 x86-based platforms. The x86 defaults are as follows:
+The "default platform calling convention" is generally unambiguous for all supported platforms except 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>