Nikolay Sivov (@nsivov) commented about dlls/ntdll/ntdll_misc.h:
#define NTDLL_TLS_ERRNO 16 /* TLS slot for _errno() */
+/* undocumented */ +#define NTDLL_STACK_FRAME_ALLOCATED 0x8 /* For RtlActivateActivationContextEx()/RtlFreeActivationContextStack() */
#ifdef __i386__
Comment is misleading, this flag is not used with those functions directly. Just mention _STACK_FRAME.Flags field. Less important suggestion would be to use an enum instead, but it's up to you.