DOC: lua: Documentation about some entry missing

The parameter "value" of the function TXN.set_var() was not documented.

This is a regression from the commit 85d79c94a93211b156ec644e164b178f5d7101fa.
This patch must be backported in 1.7
diff --git a/doc/lua-api/index.rst b/doc/lua-api/index.rst
index 26449a2..22c0538 100644
--- a/doc/lua-api/index.rst
+++ b/doc/lua-api/index.rst
@@ -1521,6 +1521,8 @@
 
   :param class_txn txn: The class txn object containing the data.
   :param string var: The variable name according with the HAProxy variable syntax.
+  :param type value: The value associated to the variable. The type can be string or
+                     integer.
 
 .. js:function:: TXN.unset_var(TXN, var)