Add recommendation to update systems before installing ROS 2.

Installing ROS 2 packages without updating the system first can lead to
conflicts due to package version differences.

This is generally useful but of particular importance on Ubuntu Jammy: ros2/ros2#1272

source/Installation/RHEL-Install-RPMs.rst

@@ -56,6 +56,13 @@ DNF may prompt you to verify the GPG key, which should match the location ``http
 Install ROS 2 packages
 ----------------------
 
+ROS 2 packages are built on frequently updated RHEL systems.
+It is always recommended that you ensure your system is up to date before installing new packages.
+
+.. code-block:: bash
+
+   sudo dnf update
+
 Desktop Install (Recommended): ROS, RViz, demos, tutorials.
 
 .. code-block:: bash

source/Installation/Ubuntu-Install-Debians.rst

@@ -47,6 +47,13 @@ Update your apt repository caches after setting up the repositories.
 
    sudo apt update
 
+ROS 2 packages are built on frequently updated Ubuntu systems.
+It is always recommended that you ensure your system is up to date before installing new packages.
+
+.. code-block:: bash
+
+   sudo apt upgrade
+
 Desktop Install (Recommended): ROS, RViz, demos, tutorials.
 
 .. code-block:: bash