KVM: clarify the idea of kvm_dirty_regs

This patch clarifies that kvm_dirty_regs are just a hint to the kernel and
that the kernel might just ignore some flags and sync the values (like done for
acrs and gprs now).
Signed-off-by: NDavid Hildenbrand <dahi@linux.vnet.ibm.com>
Reviewed-by: NCornelia Huck <cornelia.huck@de.ibm.com>
Signed-off-by: NChristian Borntraeger <borntraeger@de.ibm.com>

Documentation/virtual/kvm/api.txt

@@ -2861,6 +2861,10 @@ kvm_valid_regs for specific bits. These bits are architecture specific
 and usually define the validity of a groups of registers. (e.g. one bit
  for general purpose registers)
 
+Please note that the kernel is allowed to use the kvm_run structure as the
+primary storage for certain register types. Therefore, the kernel may use the
+values in kvm_run even if the corresponding bit in kvm_dirty_regs is not set.
+
 };