Update ilm-settings.asciidoc (elastic#51577)

Author: ppf2

docs/reference/settings/ilm-settings.asciidoc

@@ -43,4 +43,4 @@ the index creation will fail.
 `index.lifecycle.origination_date`::
 The timestamp that will be used to calculate the index age for its phase
 transitions. This allows the users to create an index containing old data and
-use the original creation date of the old data to calculate the index age.
+use the original creation date of the old data to calculate the index age.  Must be a long (Unix epoch) value.