out_es: Document Time_Key_Nanos option (#377)

Documents new feature added in fluent/fluent-bit#2544

Signed-off-by: Neal Turett <nturett@evoforge.org>

pipeline/outputs/elasticsearch.md

@@ -24,6 +24,7 @@ The **es** output plugin, allows to ingest your records into a [Elasticsearch](h
 | Logstash\_DateFormat | Time format \(based on [strftime](http://man7.org/linux/man-pages/man3/strftime.3.html)\) to generate the second part of the Index name. | %Y.%m.%d |
 | Time\_Key | When Logstash\_Format is enabled, each record will get a new timestamp field. The Time\_Key property defines the name of that field. | @timestamp |
 | Time\_Key\_Format | When Logstash\_Format is enabled, this property defines the format of the timestamp. | %Y-%m-%dT%H:%M:%S |
+| Time\_Key\_Nanos | When Logstash\_Format is enabled, enabling this property sends nanosecond precision timestamps. | Off |
 | Include\_Tag\_Key | When enabled, it append the Tag name to the record. | Off |
 | Tag\_Key | When Include\_Tag\_Key is enabled, this property defines the key name for the tag. | \_flb-key |
 | Generate\_ID | When enabled, generate `_id` for outgoing records. This prevents duplicate records when retrying ES. | Off |