Add note about negative epoch times (elastic#61379)

This commit adds a reminder to date type documentation that negative
epoch times are not supported.

relates elastic#40983

Author: rjernst

docs/reference/mapping/types/date.asciidoc

@@ -10,6 +10,9 @@ JSON doesn't have a date data type, so dates in Elasticsearch can either be:
 * a long number representing _milliseconds-since-the-epoch_.
 * an integer representing _seconds-since-the-epoch_.
 
+NOTE: Values for _milliseconds-since-the-epoch_ and _seconds-since-the-epoch_
+must be non-negative. Use a formatted date to represent dates before 1970.
+
 Internally, dates are converted to UTC (if the time-zone is specified) and
 stored as a long number representing milliseconds-since-the-epoch.