MEDIUM: add set-priority-class and set-priority-offset

This adds the set-priority-class and set-priority-offset actions to
http-request and tcp-request content. At this point they are not used
yet, which is the purpose of the next commit, but all the logic to
set and clear the values is there.
diff --git a/doc/lua-api/index.rst b/doc/lua-api/index.rst
index ee9dab5..0c79766 100644
--- a/doc/lua-api/index.rst
+++ b/doc/lua-api/index.rst
@@ -1769,6 +1769,24 @@
   :param class_txn txn: The class txn object containing the data.
   :param integer mark: The mark value.
 
+.. js:function:: TXN.set_priority_class(txn, prio)
+
+  This function adjusts the priority class of the transaction. The value should
+  be within the range -2047..2047. Values outside this range will be
+  truncated.
+
+  See the HAProxy configuration.txt file keyword "http-request" action
+  "set-priority-class" for details.
+
+.. js:function:: TXN.set_priority_offset(txn, prio)
+
+  This function adjusts the priority offset of the transaction. The value
+  should be within the range -524287..524287. Values outside this range will be
+  truncated.
+
+  See the HAProxy configuration.txt file keyword "http-request" action
+  "set-priority-offset" for details.
+
 .. _socket_class:
 
 Socket class