InstantFormatter fails on delegate failures

After LOG4J2-3075 was implemented, the JsonTemplateLayout timestamp resolver documentation should be updated to reflect the new feature.

It's especially important to do so since the second fragment identifier used here ('S') is different from the one described in, e.g. https://logging.apache.org/log4j/2.x/manual/layouts.html (which is 'n'). I don't have enough experience with Log4j to determine if this inconsistency is a bug or not, but an entry in the JsonTemplateLayout  documentation would help.