qmp-commands.hx: Update the supported 'transaction' operations

Although the canonical source of reference for QMP commands is
qapi-schema.json, for consistency's sake, update qmp-commands.hx to
state the list of supported transactionable operations, namely:

    drive-backup
    blockdev-backup
    blockdev-snapshot-internal-sync
    abort
    block-dirty-bitmap-add
    block-dirty-bitmap-clear

Also update the possible values for the "type" action array.
Signed-off-by: NKashyap Chamarthy <kchamart@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>

qmp-commands.hx

@@ -1270,11 +1270,22 @@ SQMP
 transaction
 -----------
 
-Atomically operate on one or more block devices.  The only supported operations
-for now are drive-backup, internal and external snapshotting.  A list of
-dictionaries is accepted, that contains the actions to be performed.
-If there is any failure performing any of the operations, all operations
-for the group are abandoned.
+Atomically operate on one or more block devices.  Operations that are
+currently supported:
+
+    - drive-backup
+    - blockdev-backup
+    - blockdev-snapshot-sync
+    - blockdev-snapshot-internal-sync
+    - abort
+    - block-dirty-bitmap-add
+    - block-dirty-bitmap-clear
+
+Refer to the qemu/qapi-schema.json file for minimum required QEMU
+versions for these operations.  A list of dictionaries is accepted,
+that contains the actions to be performed.  If there is any failure
+performing any of the operations, all operations for the group are
+abandoned.
 
 For external snapshots, the dictionary contains the device, the file to use for
 the new snapshot, and the format.  The default format, if not specified, is
@@ -1301,8 +1312,12 @@ it later with qemu-img or other command.
 Arguments:
 
 actions array:
-    - "type": the operation to perform.  The only supported
-      value is "blockdev-snapshot-sync". (json-string)
+    - "type": the operation to perform (json-string).  Possible
+              values: "drive-backup", "blockdev-backup",
+                      "blockdev-snapshot-sync",
+                      "blockdev-snapshot-internal-sync",
+                      "abort", "block-dirty-bitmap-add",
+                      "block-dirty-bitmap-clear"
     - "data": a dictionary.  The contents depend on the value
       of "type".  When "type" is "blockdev-snapshot-sync":
       - "device": device name to snapshot (json-string)