Bug 1509900 - Improve documentation for taskcluster-load-image. r=dustin

Differential Revision: https://phabricator.services.mozilla.com/D12932

--HG--
extra : moz-landing-system : lando

taskcluster/mach_commands.py

@@ -421,7 +421,8 @@ class TaskClusterImagesProvider(MachCommandBase):
             self.virtualenv_manager.install_pip_package('zstandard==0.9.0')
 
     @Command('taskcluster-load-image', category="ci",
-             description="Load a pre-built Docker image")
+             description="Load a pre-built Docker image. Note that you need to "
+                         "have docker installed and running for this to work.")
     @CommandArgument('--task-id',
                      help="Load the image at public/image.tar.zst in this task, "
                           "rather than searching the index")