qmp-commands: move 'block-dirty-bitmap-clear' doc to schema

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
diff --git a/qapi/block-core.json b/qapi/block-core.json
index 13a9742..d8c6706 100644
--- a/qapi/block-core.json
+++ b/qapi/block-core.json
@@ -1611,13 +1611,22 @@
 ##
 # @block-dirty-bitmap-clear:
 #
-# Clear (reset) a dirty bitmap on the device
+# Clear (reset) a dirty bitmap on the device, so that an incremental
+# backup from this point in time forward will only backup clusters
+# modified after this clear operation.
 #
 # Returns: nothing on success
 #          If @node is not a valid block device, DeviceNotFound
 #          If @name is not found, GenericError with an explanation
 #
 # Since: 2.4
+#
+# Example:
+#
+# -> { "execute": "block-dirty-bitmap-clear",
+#      "arguments": { "node": "drive0", "name": "bitmap0" } }
+# <- { "return": {} }
+#
 ##
 { 'command': 'block-dirty-bitmap-clear',
   'data': 'BlockDirtyBitmap' }