DOCS-14846 non system user ulimit

Author: jason-price-mongodb

source/reference/ulimit.txt

@@ -150,6 +150,27 @@ change to system limits made using ``ulimit`` may revert following
 a system restart. Check your distribution and operating
 system documentation for more information.
 
+You should typically start :binary:`~bin.mongod` using ``systemctl``,
+which uses the ``ulimit`` settings:
+
+.. code-block:: bash
+
+   systemctl start mongod.service
+
+If you *do not* start :binary:`~bin.mongod` using ``systemctl``,
+``systemd`` overrides some of the ``ulimit`` settings. For example, if
+you start :binary:`~bin.mongod` as shown in the following command, then
+a user slice (such as ``user-1000.slice``) ``systemd`` setting is used:
+
+.. code-block:: bash
+
+   mongod --config ~/mongod.conf
+
+.. note::
+
+   A ``systemd`` user slice limits the resources for the user's
+   processes.
+
 macOS
 `````