Enhance docs for backup and software center

- Clarify restrictions on restoring to specific cluster nodes in
  backup.rst.
- Update software_center.rst to detail cloning limitations due to
  application policies, resource constraints, or exclusive resource
  requirements.
- Add a reference for selective content restore in backup.rst.

backup.rst

@@ -150,9 +150,14 @@ available.
   app`` checkbox becomes visible. If enabled, the existing instance will
   be removed upon restore.
 * Select an older backup snapshot if the latest one is not preferable.
-* Select the target node.
+* Select the target node. Note that in some cases, restoring to certain
+  cluster nodes may be restricted due to application policies or node
+  resource limitations.
 * Click on the :guilabel:`Restore` button.
 
+
+.. _selective-content-restore:
+
 Selective content restore
 =========================
 

software_center.rst

@@ -224,11 +224,11 @@ source one.
 
 The cluster node where the destination instance runs can be the same as
 the source instance. Generally, there should be no limitation on running
-multiple application instances on the same node. In some cases, the
-services provided by the instance might require exclusive access to a
-particular system resource, such as binding a fixed TCP port number. In
-this case starting additional instances of the same service becomes
-impossible.
+multiple application instances on the same node. However, in some cases,
+cloning to certain cluster nodes may be restricted due to application
+policies, node resource limitations, or because the instance requires
+exclusive access to a system resource, such as binding a fixed TCP port
+number.
 
 In a nutshell the clone procedure: