Docs: Added note about upgrading from 1.x to 5.x

docs/reference/migration/index.asciidoc

@@ -8,11 +8,14 @@ your application from one version of Elasticsearch to another.
 
 As a general rule:
 
-* Migration between major versions -- e.g. `1.x` to `2.x` --
+* Migration between minor versions -- e.g. `5.x` to `5.y` -- can be
+  performed by <<rolling-upgrades,upgrading one node at a time>>.
+
+* Migration between consecutive major versions -- e.g. `2.x` to `5.x` --
   requires a <<restart-upgrade,full cluster restart>>.
 
-* Migration between minor versions -- e.g. `1.x` to `1.y` -- can be
-  performed by <<rolling-upgrades,upgrading one node at a time>>.
+* Migration between non-consecutive major versions -- e.g. `1.x` to `5.x` --
+  is not supported.
 
 See <<setup-upgrade>> for more info.
 --