Document the origin of the DateProcessor format literals (#99388)

modules/ingest-common/src/main/java/org/elasticsearch/ingest/common/DateFormat.java

@@ -126,6 +126,10 @@ enum DateFormat {
     abstract Function<String, ZonedDateTime> getFunction(String format, ZoneId timezone, Locale locale);
 
     static DateFormat fromString(String format) {
+        // note: the ALL_CAPS format names here (UNIX_MS, etc) are present for historical reasons:
+        // they are the format literals that are supported by the logstash date filter plugin
+        // (see https://www.elastic.co/guide/en/logstash/current/plugins-filters-date.html#plugins-filters-date-match).
+        // don't extend this list with new special keywords (unless logstash has grown the same keyword).
         return switch (format) {
             case "ISO8601" -> Iso8601;
             case "UNIX" -> Unix;

modules/ingest-common/src/yamlRestTest/resources/rest-api-spec/test/ingest/30_date_processor.yml

@@ -152,6 +152,13 @@ teardown:
                   "target_field" : "date_target_7",
                   "formats" : [ "ISO8601" ]
                 }
+              },
+              {
+                "date" : {
+                  "field" : "date_source_8",
+                  "target_field" : "date_target_8",
+                  "formats" : [ "epoch_second" ]
+                }
               }
             ]
           }
@@ -162,7 +169,16 @@ teardown:
         index: test
         id: "1"
         pipeline: "my_pipeline"
-        body: { date_source_1: "2018-02-05T13:44:56.657+0100", date_source_2: "2017-04-04 13:43:09 +0200", date_source_3: "10/Aug/2018:09:45:56 +0200", date_source_4: "1", date_source_5: "1", date_source_6: "4000000050d506482dbdf024", date_source_7: "2018-02-05T13:44:56.657+0100" }
+        body: {
+          date_source_1: "2018-02-05T13:44:56.657+0100",
+          date_source_2: "2017-04-04 13:43:09 +0200",
+          date_source_3: "10/Aug/2018:09:45:56 +0200",
+          date_source_4: "1",
+          date_source_5: "1",
+          date_source_6: "4000000050d506482dbdf024",
+          date_source_7: "2018-02-05T13:44:56.657+0100",
+          date_source_8: "1688548995.987654321"
+      }
 
   - do:
       get:
@@ -182,6 +198,7 @@ teardown:
   - match: { _source.date_target_6: "2012-12-22T01:00:46.767Z" }
   - match: { _source.date_source_7: "2018-02-05T13:44:56.657+0100" }
   - match: { _source.date_target_7: "2018-02-05T12:44:56.657Z" }
+  - match: { _source.date_target_8: "2023-07-05T09:23:15.987Z" }
 
 
 ---