[DOCS] Add deprecation docs for fractional byte size values

docs/reference/migration/index.asciidoc

@@ -28,6 +28,7 @@ For more information about {minor-version},
 see the <<release-highlights>> and <<es-release-notes>>.
 For information about how to upgrade your cluster, see <<setup-upgrade>>.
 
+* <<migrating-7.15,Migrating to 7.15>>
 * <<breaking-changes-7.14,Migrating to 7.14>>
 * <<breaking-changes-7.13,Migrating to 7.13>>
 * <<breaking-changes-7.12,Migrating to 7.12>>
@@ -46,6 +47,7 @@ For information about how to upgrade your cluster, see <<setup-upgrade>>.
 
 --
 
+include::migrate_7_15.asciidoc[]
 include::migrate_7_14.asciidoc[]
 include::migrate_7_13.asciidoc[]
 include::migrate_7_12.asciidoc[]

docs/reference/migration/migrate_7_15.asciidoc

@@ -0,0 +1,74 @@
+[[migrating-7.15]]
+== Migrating to 7.15
+++++
+<titleabbrev>7.15</titleabbrev>
+++++
+
+This section discusses the changes that you need to be aware of when migrating
+your application to {es} 7.15.
+
+See also <<release-highlights>> and <<es-release-notes>>.
+
+* <<breaking_715_settings_deprecations>>
+
+////
+//NOTE: The notable-breaking-changes tagged regions are re-used in the
+//Installation and Upgrade Guide
+
+[discrete]
+[[breaking-changes-7.15]]
+=== Breaking changes
+
+The following changes in {es} 7.15 might affect your applications
+and prevent them from operating normally.
+Before upgrading to 7.15, review these changes and take the described steps
+to mitigate the impact.
+
+NOTE: Breaking changes introduced in minor versions are
+normally limited to security and bug fixes.
+Significant changes in behavior are deprecated in a minor release and
+the old behavior is supported until the next major release.
+To find out if you are using any deprecated functionality,
+enable <<deprecation-logging, deprecation logging>>.
+
+// tag::notable-breaking-changes[]
+// end::notable-breaking-changes[]
+////
+
+[discrete]
+[[deprecated-7.15]]
+=== Deprecations
+
+The following functionality has been deprecated in {es} 7.15 and will be removed
+in 8.0. While this won't have an immediate impact on your applications, we
+strongly encourage you take the described steps to update your code after
+upgrading to 7.15.
+
+NOTE: Significant changes in behavior are deprecated in a minor release and the
+old behavior is supported until the next major release. To find out if you are
+using any deprecated functionality, enable <<deprecation-logging, deprecation
+logging>>.
+
+// tag::notable-breaking-changes[]
+[discrete]
+[[breaking_715_settings_deprecations]]
+==== Settings deprecations
+
+[[deprecate-fractional-byte-settings]]
+.Fractional byte size values are deprecated.
+[%collapsible]
+====
+*Details* +
+In 6.2, we deprecated support for fractional byte size values, such as `23.5pb`.
+In 8.0, {es} will reject fractional byte size values for:
+
+* Cluster settings
+* Index settings
+* Cluster state metadata, such as an {ilm-init} policy, that support byte size
+values
+
+*Impact* +
+To avoid deprecation warnings, discontinue use of fractional byte size values in
+your configurations. Update any existing configurations to use whole values.
+====
+// end::notable-breaking-changes[]