Add tt pack DEB/RPM content description (#4206)

Resolves #4078

Author: p7nov

doc/book/admin/instance_config.rst

@@ -151,14 +151,23 @@ The application's layout looks similar to the one defined when :ref:`developing
 
 -   ``tt.yaml``: a ``tt`` configuration file.
 
+.. note::
 
+    In DEB/PRM packages generated by :ref:`tt pack <tt-pack>`, there are also ``.service``
+    unit files for each packaged application.
+
+.. _admin-instance_config-deploy-app:
+
+Deploying the application
+-------------------------
 
 .. _admin-instances_to_run:
 
 Instances to run
 ~~~~~~~~~~~~~~~~
 
-One more difference for a deployed application is the content of the ``instances.yaml`` file that specifies instances to run in the current environment.
+When deploying a distributed cluster application from a `.tar.gz` archive, you can
+define instances to run on each machine by changing the content of the ``instances.yaml`` file.
 
 -   On the developer's machine, this file might include all the instances defined in the cluster configuration.
 
@@ -200,5 +209,22 @@ One more difference for a deployed application is the content of the ``instances
         storage-a-002:
         storage-b-002:
 
-
 The :ref:`Starting and stopping instances <admin-start_stop_instance>` section describes how to start and stop Tarantool instances.
+
+.. _admin-deploy-rpm-deb:
+
+DEB and RPM packages
+~~~~~~~~~~~~~~~~~~~~
+
+Tarantool applications installed from DEB and RPM packages built with :ref:`tt pack <tt-pack>`
+can run as ``systemd`` services. They run on behalf of the ``tarantool`` system user.
+It is created automatically during the package installation.
+
+By default, the application artifacts are placed in the following directories:
+
+-   ``/var/lib/tarantool/sys_env`` -- application data
+-   ``/var/log/tarantool/sys_env`` -- logs
+-   ``/var/run/tarantool/sys_env`` -- runtime artifacts
+
+If you want to change these directories, make sure that the ``tarantool`` user
+has enough permissions on the directories you use.

doc/reference/tooling/tt_cli/pack.rst

@@ -13,13 +13,27 @@ Packaging the application
 -   ``deb``: create a DEB package.
 -   ``rpm``: create an RPM package.
 
-The command below creates a DEB package for an application:
+.. _tt-pack-example:
 
-    .. code-block:: console
+Example: a DEB package
+----------------------
+
+The command below creates a DEB package with all applications from the current ``tt``
+environment:
+
+.. code-block:: console
 
-        $ tt pack deb
+    $ tt pack deb
 
 This command generates a ``.deb`` file whose name depends on the environment directory name and the operating system architecture, for example, ``test-env_0.1.0.0-1_x86_64.deb``.
+The package contains the following files:
+
+-   The content of the application directories: source files, resources, dependencies.
+-   ``tt`` environment files: ``tarantool`` and ``tt`` executables, ``tt.yaml`` configuration file,
+    external modules, headers.
+-   ``.service`` unit files that allow running applications as ``systemd`` services
+    (a separate file for each application).
+
 You can also pass various :ref:`options <tt-pack-options>` to the ``tt pack`` command to adjust generation properties, for example, customize a bundle name, choose which artifacts should be included, specify the required application dependencies.