doc: add documentation for TlsGetCertSerial Lua function

doc/userguide/output/lua-output.rst

@@ -457,6 +457,22 @@ Example:
   end
 
 
+TlsGetCertSerial
+~~~~~~~~~~~~~~~~
+
+Get TLS certificate serial number through TlsGetCertSerial.
+
+Example:
+
+::
+
+  function log (args)
+      serial = TlsGetCertSerial()
+      if serial then
+          -- do something
+      end
+  end
+
 SSH
 ---