[DOCS] [7.x] Document deprecation of storing nanosecond resolution on date field (#79554)

relates #78921

Author: pgomulka

docs/reference/migration/migrate_7_16.asciidoc

@@ -88,7 +88,7 @@ logging>>.
 ====
 *Details* +
 In SAML, Identity Providers (IdPs) can either be explicitly configured to
-release a `NameID` with a specific format, or configured to attempt to conform 
+release a `NameID` with a specific format, or configured to attempt to conform
 with the requirements of a Service Provider (SP). The SP declares its
 requirements in the `NameIDPolicy` element of a SAML Authentication Request.
 In {es}, the `nameid_format` SAML realm setting controls the `NameIDPolicy`
@@ -103,9 +103,9 @@ IdP. If you want to retain the previous behavior, set `nameid_format` to
 
 *Impact* +
 If you currently don't configure `nameid_format` explicitly, it's possible
-that your IdP will reject authentication requests from {es} because the requests 
+that your IdP will reject authentication requests from {es} because the requests
 do not specify a `NameID` format (and your IdP is configured to expect one).
-This mismatch can result in a broken SAML configuration. If you're unsure whether 
+This mismatch can result in a broken SAML configuration. If you're unsure whether
 your IdP is explicitly configured to use a certain `NameID` format and you want to retain current behavior
 , try setting `nameid_format` to `urn:oasis:names:tc:SAML:2.0:nameid-format:transient` explicitly.
 ====
@@ -347,4 +347,19 @@ cache do not expire.
 To override the defaults, configure the `script.cache.max_size`,
 `script.max_compilations_rate`, and `script.cache.expire` settings.
 ====
+
+.Attempting to store nanosecond resolution in a `date` field is deprecated.
+[%collapsible]
+====
+*Details* +
+Attempting to store a nanosecond resolution in a {ref}/date.html[`date`] field is deprecated.
+While previously allowed, these attempts always resulted in resolution loss.
+A `date` field can only store up to millisecond resolutions.
+
+*Impact* +
+If you attempt to store a nanosecond resolution in a `date` type field, {es} will
+emit a deprecation warning. To avoid deprecation warnings, use a
+{ref}/date_nanos.html[`date_nanos`] field instead.
+====
+
 // end::notable-breaking-changes[]