xen/elfnote: Specify ELF Notes are x86-specific

The Xen ELF Notes are only used with x86.  libelf's elf_xen_note_check()
exits early for ARM binaries with "ELF: Not bothering with notes on
ARM".

Add a comment to the top of elfnote.h specifying that Notes are only used
with x86 binaries.  This is to avoid adding disclaimers for individual
notes.

Signed-off-by: Jason Andryuk <jason.andryuk@amd.com>
Acked-by: Jan Beulich <jbeulich@suse.com>

xen/include/public/elfnote.h

@@ -24,6 +24,8 @@
  *
  * String values (for non-legacy) are NULL terminated ASCII, also known
  * as ASCIZ type.
+ *
+ * Xen only uses ELF Notes contained in x86 binaries.
  */
 
 /*