docs: Note on version selection in Cube Cloud

Author: igorlukanin

docs/pages/product/deployment/cloud/deployments.mdx

@@ -127,6 +127,9 @@ General → Cube version</Btn>:
   highlight="inset(36% 41% 34% 21% round 10px)"
 />
 
+You can select a specific version in the drop-down. Only versions that have been used by
+a deployment are available in the list.
+
 There's an option to <Btn>Upgrade automatically to new patch versions</Btn>, which is
 available for both update channels. When it's turned on, an upgrade will occur if a new
 [patch version][link-semver] is available in the selected channel. To trigger an upgrade,