[DOCS] Clarify upgrade paths (#53417)

Unsupported upgrade paths:
- `6.8 to 7.0`.
Supported, but requires a full cluster restart:
- `6.0–6.7 directly to 7.x`

docs/reference/upgrade.asciidoc

@@ -47,10 +47,18 @@ a|
 
 [WARNING]
 ====
-The following upgrade paths are *not* supported:
+The upgrade path from 6.8 to 7.0 is *not* supported (both full cluster restart and rolling upgrade).
+====
 
-* 6.8 to 7.0.
-* 6.7 to 7.1.–{version}.
+To upgrade directly to {version} from 6.7 or earlier, you must shut down the
+cluster, install {version}, and restart. For more information, see
+<<restart-upgrade, Full cluster restart upgrade>>.
+
+[WARNING]
+====
+In-place downgrades to earlier versions are *not* supported. To downgrade to an
+earlier version, <<snapshots-restore-snapshot,restore a snapshot>> taken prior
+to the version upgrade.
 ====
 
 {es} can read indices created in the previous major version. If you
@@ -64,10 +72,6 @@ When upgrading to a new version of {es}, you need to upgrade each
 of the products in your Elastic Stack. For more information, see the
 {stack-ref}/upgrading-elastic-stack.html[Elastic Stack Installation and Upgrade Guide].
 
-To upgrade directly to {version} from 6.7 or earlier, you must shut down the
-cluster, install {version}, and restart. For more information, see
-<<restart-upgrade, Full cluster restart upgrade>>.
-
 --
 
 include::upgrade/rolling_upgrade.asciidoc[]