KVM: add comments for kvm_debug_exit_arch struct

Bring into line with the comments for the other structures and their
KVM_EXIT_* cases. Also update api.txt to reflect use in kvm_run
documentation.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: David Hildenbrand <dahi@linux.vnet.ibm.com>
Reviewed-by: Andrew Jones <drjones@redhat.com>
Acked-by: Christoffer Dall <christoffer.dall@linaro.org>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
diff --git a/Documentation/virtual/kvm/api.txt b/Documentation/virtual/kvm/api.txt
index a7926a9..9f746ea 100644
--- a/Documentation/virtual/kvm/api.txt
+++ b/Documentation/virtual/kvm/api.txt
@@ -3111,11 +3111,13 @@
 where kvm expects application code to place the data for the next
 KVM_RUN invocation (KVM_EXIT_IO_IN).  Data format is a packed array.
 
+		/* KVM_EXIT_DEBUG */
 		struct {
 			struct kvm_debug_exit_arch arch;
 		} debug;
 
-Unused.
+If the exit_reason is KVM_EXIT_DEBUG, then a vcpu is processing a debug event
+for which architecture specific information is returned.
 
 		/* KVM_EXIT_MMIO */
 		struct {