[doc] Document deprecation of storing nanosecond resolution on date

field

relates elastic#78921

Author: pgomulka

docs/reference/migration/migrate_8_0/mappings.asciidoc

@@ -89,6 +89,21 @@ For a detailed migration guide, see the {ref}/migrate-to-java-time.html[Java
 time migration guide].
 ====
 
+.Attempting to store nanosecond resolution on date field is deprecated
+[%collapsible]
+====
+*Details* +
+Attempting to store a nanosecond resolution on a date field is deprecated.
+This behaviour was allowed so far, but was always resulting in resolution loss.
+The field of type `date` can only store up to milliseconds resolution.
+
+
+*Impact* +
+A deprecation warning will be emitted when attempting to store a nanosecond resolution
+on `date` type field. `date_nanos` should be used instead
+====
+
+
 [[geo-shape-strategy]]
 .Several `geo_shape` mapping parameters have been removed.
 [%collapsible]