[DOCS][ESQL] Add link to date_trunc docs, update tests

x-pack/plugin/esql/qa/testFixtures/src/main/resources/meta.csv-spec

@@ -153,7 +153,7 @@ date_diff     |[unit, startTimestamp, endTimestamp]|["keyword|text", date, date]
 date_extract  |[datePart, date]                    |["keyword|text", date]                                                                                                            |[Part of the date to extract.  Can be: `aligned_day_of_week_in_month`\, `aligned_day_of_week_in_year`\, `aligned_week_of_month`\, `aligned_week_of_year`\, `ampm_of_day`\, `clock_hour_of_ampm`\, `clock_hour_of_day`\, `day_of_month`\, `day_of_week`\, `day_of_year`\, `epoch_day`\, `era`\, `hour_of_ampm`\, `hour_of_day`\, `instant_seconds`\, `micro_of_day`\, `micro_of_second`\, `milli_of_day`\, `milli_of_second`\, `minute_of_day`\, `minute_of_hour`\, `month_of_year`\, `nano_of_day`\, `nano_of_second`\, `offset_seconds`\, `proleptic_month`\, `second_of_day`\, `second_of_minute`\, `year`\, or `year_of_era`. Refer to https://docs.oracle.com/javase/8/docs/api/java/time/temporal/ChronoField.html[java.time.temporal.ChronoField] for a description of these values.  If `null`\, the function returns `null`., Date expression. If `null`\, the function returns `null`.]
 date_format   |[dateFormat, date]                  |["keyword|text", date]                                                                                                            |[Date format (optional).  If no format is specified\, the `yyyy-MM-dd'T'HH:mm:ss.SSSZ` format is used. If `null`\, the function returns `null`., Date expression. If `null`\, the function returns `null`.]
 date_parse    |[datePattern, dateString]           |["keyword|text", "keyword|text"]                                                                                                  |[The date format. Refer to the https://docs.oracle.com/en/java/javase/14/docs/api/java.base/java/time/format/DateTimeFormatter.html[`DateTimeFormatter` documentation] for the syntax. If `null`\, the function returns `null`., Date expression as a string. If `null` or an empty string\, the function returns `null`.]
-date_trunc    |[interval, date]                    |["date_period|time_duration", date]                                                                                               |[Interval; expressed using the timespan literal syntax., Date expression]
+date_trunc    |[interval, date]                    |["date_period|time_duration", date]                                                                                               |[Interval; expressed using the <<esql-timespan-literals\,timespan literal syntax>>., Date expression]
 e             |null                                |null                                                                                                                              |null
 ends_with     |[str, suffix]                       |["keyword|text", "keyword|text"]                                                                                                  |[String expression. If `null`\, the function returns `null`., String expression. If `null`\, the function returns `null`.]
 exp           |number                              |"double|integer|long|unsigned_long"                                                                                               |Numeric expression. If `null`, the function returns `null`.

x-pack/plugin/esql/src/main/java/org/elasticsearch/xpack/esql/expression/function/scalar/date/DateTrunc.java

@@ -70,7 +70,7 @@ public DateTrunc(
         @Param(
             name = "interval",
             type = { "date_period", "time_duration" },
-            description = "Interval; expressed using the timespan literal syntax."
+            description = "Interval; expressed using the <<esql-timespan-literals,timespan literal syntax>>."
         ) Expression interval,
         @Param(name = "date", type = { "date" }, description = "Date expression") Expression field
     ) {