Add --log-root documentation.

Author: jglogan

docs/command-reference.md

@@ -945,13 +945,14 @@ Starts the container services and (optionally) installs a default kernel. It wil
 **Usage**
 
 ```bash
-container system start [--app-root <app-root>] [--install-root <install-root>] [--enable-kernel-install] [--disable-kernel-install] [--debug]
+container system start [--app-root <app-root>] [--install-root <install-root>] [--log-root <log-root>] [--enable-kernel-install] [--disable-kernel-install] [--debug]
 ```
 
 **Options**
 
 *   `-a, --app-root <app-root>`: Path to the root directory for application data
 *   `--install-root <install-root>`: Path to the root directory for application executables and plugins
+*   `--log-root <log-root>`: Path to the root directory for log data, using macOS log facility if not set
 *   `--enable-kernel-install/--disable-kernel-install`: Specify whether the default kernel should be installed or not (default: prompt user)
 
 ### `container system stop`