Add systemd unit parameterization to tt pack docs (#4332)

Resolves #4201

Author: p7nov

doc/reference/tooling/tt_cli/pack.rst

@@ -36,6 +36,46 @@ The package contains the following files:
 
 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.
 
+.. _tt-pack-systemd:
+
+systemd unit parameters
+-----------------------
+
+You can customize your application's systemd unit file generated by ``tt pack``.
+To add parameters to the unit file, define them in a YAML file named ``systemd-unit-params.yml``
+in the :ref:`application directory <admin-instance_config-develop-app>`.
+
+.. code-block:: console
+
+    $ tt pack rpm # unit file with parameters from systemd-unit-params.yml if it exists
+
+You can also pass unit parameters from an arbitrary file by adding the ``--unit-params-file``
+option to the ``tt pack`` call:
+
+.. code-block:: console
+
+    $ tt pack rpm --unit-params-file my-params.yml # unit file with parameters from my-params.yml
+
+.. important::
+
+   The ``systemd-unit-params.yml`` file has a higher priority than the ``--unit-params-file`` option.
+    If this file exists, it overrides parameters from the file passed in the option.
+
+``tt pack`` supports the following systemd unit parameters:
+
+-   ``FdLimit`` -- the number of open file descriptors (``LimitNOFile`` in the unit file).
+-   ``instance-env`` -- a list of environment variables in the ``<VAR_NAME>: <VALUE>`` format.
+    Each list item adds an ``Environment=<VAR_NAME>=<VALUE>`` line to the unit file.
+
+An example of the ``systemd-unit-params.yml`` file:
+
+.. code-block:: yaml
+
+    FdLimit: 200
+    instance-env:
+      INSTANCE: "inst:%i"
+      TARANTOOL_WORKDIR: "/tmp"
+
 
 .. _tt-pack-options:
 
@@ -145,6 +185,10 @@ Options
     Specify a Tarantool version for packaging in a Docker container.
     For use with ``--use-docker`` only.
 
+..  option:: --unit-params-file
+
+    The path to a file with custom :ref:`systemd unit parameters <tt-pack-systemd>`.
+
 ..  option:: --use-docker
 
     Build a package in an Ubuntu 18.04 Docker container. To specify a Tarantool
@@ -166,6 +210,10 @@ Options
 
     Include Tarantool and ``tt`` binaries in a bundle.
 
+..  option:: --with-tarantool-deps
+
+    Add Tarantool and ``tt`` as package dependencies.
+
 ..  option:: --without-binaries
 
     Don't include Tarantool and ``tt`` binaries in a bundle.