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>

Documentation/virtual/kvm/api.txt

@@ -3111,11 +3111,13 @@ data_offset describes where the data is located (KVM_EXIT_IO_OUT) or
 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 {

include/uapi/linux/kvm.h

@@ -237,6 +237,7 @@ struct kvm_run {
 			__u32 count;
 			__u64 data_offset; /* relative to kvm_run start */
 		} io;
+		/* KVM_EXIT_DEBUG */
 		struct {
 			struct kvm_debug_exit_arch arch;
 		} debug;
@@ -285,6 +286,7 @@ struct kvm_run {
 			__u32 data;
 			__u8  is_write;
 		} dcr;
+		/* KVM_EXIT_INTERNAL_ERROR */
 		struct {
 			__u32 suberror;
 			/* Available with KVM_CAP_INTERNAL_ERROR_DATA: */
@@ -295,6 +297,7 @@ struct kvm_run {
 		struct {
 			__u64 gprs[32];
 		} osi;
+		/* KVM_EXIT_PAPR_HCALL */
 		struct {
 			__u64 nr;
 			__u64 ret;