Document the ctx._now variable in the update API

Relates to #20835

docs/reference/docs/update.asciidoc

@@ -67,7 +67,7 @@ POST test/type1/1/_update
 
 In addition to `_source`, the following variables are available through
 the `ctx` map: `_index`, `_type`, `_id`, `_version`, `_routing`,
-`_parent`.
+`_parent`, and `_now` (the current timestamp).
 
 We can also add a new field to the document: