x86/cfi: Extend {JMP,CAKK}_NOSPEC comment

With the introduction of kCFI these helpers are no longer equivalent
to C indirect calls and should be used with care.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Sami Tolvanen <samitolvanen@google.com>
Link: https://lkml.kernel.org/r/20230622144321.360957723%40infradead.org