Adds isolated instance mode description

* Adds isolated instance mode description
* The ```isolated: true``` option moves the instance to RO mode
* In this mode, various debugging & maintanance operations can be performed
* Fixes #4632

Author: AArdeev

doc/admin/troubleshoot.rst

@@ -254,6 +254,10 @@ This issue can be fixed in two ways:
 -   Manually: :ref:`reseed <replication-master-master-reseed-replica>` one master from another by removing write-ahead logs and snapshots.
 -   Programmatically: set up a :ref:`conflict resolution trigger <replication-problem_solving>`.
 
+..  NOTE::
+
+    If one of the instances must be isolated during troubleshooting, it can be put to the :ref:`isolated mode <_configuration_reference_isolated_mode>`.
+
 Then, restart replication as described in :ref:`Restarting replication <replication-master-master-resolve-conflict>`.
 
 

doc/code_snippets/snippets/config/instances.enabled/isolated_mode/config.yaml

@@ -0,0 +1,10 @@
+groups:
+  g:
+    replicasets:
+      r:
+        instances:
+          i-001: {}
+          i-002: {}
+          i-003: {}
+          i-004:
+            isolated: true

doc/reference/configuration/configuration_reference.rst

@@ -2805,6 +2805,38 @@ instances
     Any configuration parameter that can be defined in the instance :ref:`scope <configuration_scopes>`.
     For example, :ref:`iproto <configuration_reference_iproto>` and :ref:`database <configuration_reference_database>` configuration parameters defined at the instance level are applied to this instance only.
 
+..  _configuration_reference_isolated_mode:
+
+isolated mode
+------
+
+Since version :doc:`3.3.0 </release/3.3.0>`, a new ``isolated`` option is added to instance configuration.
+
+The option takes boolean values, by default it is set to ``false``.
+``isolated: true`` moves the instance it has been applied at to the **isolated** mode.
+
+The isolated mode allows the user to temporarily isolate an instance and perform maintenance activities on it. 
+
+In the isolated mode:
+
+*   The instance is moved to the read-only state
+*   iproto stops listening for new connections
+*   iproto drops all the current connections
+*   The instance is disconnected from all the replication upstreams
+*   Other replicaset members exclude the isolated instance from the replication upstreams
+
+..  NOTE::
+
+    Isolated instance can't be bootstrapped (a local snapshot is required to start).
+
+**Example**
+
+    The example below shows how to isolate an instance:
+
+    ..  literalinclude:: /code_snippets/snippets/config/instances.enabled/isolated_mode/config.yaml
+        :language: yaml
+        :dedent:
+
 ..  _configuration_reference_labels:
 
 labels