Add upgrade note about release date constraint

We don't support (and will actively block) upgrades to
chronologically-older versions. This commit adds a note in the docs
about this.

Author: DaveCTurner

docs/reference/upgrade.asciidoc

@@ -25,6 +25,18 @@ proceeding with the upgrade. For instructions, refer to
 {stack-ref}/upgrading-elastic-stack.html#prepare-to-upgrade[Prepare to upgrade
 from 7.x].
 
+[discrete]
+[[upgrade-newer-releases]]
+=== Out-of-order releases
+
+Elastic maintains several minor versions of {es} at once, so releases do not
+always happen in order of their version numbers. You can only upgrade to
+{version} if the version you are currently running has an older version number
+than {version} _and_ has an earlier release date than {version}.
+
+If you are currently running a version with an older version number but a later
+release date than {version}, wait for a newer release before upgrading.
+
 [discrete]
 [[upgrade-index-compatibility]]
 === Index compatibility