xen/memory: Clarify the XENMEM_acquire_resource ABI description

This is how similar operations already operate, compatible with the sole
implementation (in Linux), and explicitly gives us some flexibility.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Paul Durrant <paul@xen.org>

xen/include/public/memory.h

@@ -648,7 +648,8 @@ struct xen_mem_acquire_resource {
     uint32_t pad;
     /*
      * IN - the index of the initial frame to be mapped. This parameter
-     *      is ignored if nr_frames is 0.
+     *      is ignored if nr_frames is 0.  This value may be updated
+     *      over the course of the operation.
      */
     uint64_t frame;
 
@@ -664,7 +665,8 @@ struct xen_mem_acquire_resource {
      *          If -EIO is returned then the frame_list has only been
      *          partially mapped and it is up to the caller to unmap all
      *          the GFNs.
-     *          This parameter may be NULL if nr_frames is 0.
+     *          This parameter may be NULL if nr_frames is 0.  This
+     *          value may be updated over the course of the operation.
      */
     XEN_GUEST_HANDLE(xen_pfn_t) frame_list;
 };