Merge branch 'exec_warning' into 'master'

Explain why GitlabShell#exec_cmd has no tests

lib/gitlab_shell.rb

@@ -59,6 +59,7 @@ class GitlabShell
     api.allowed?(@git_cmd, @repo_name, @key_id, '_any')
   end
 
+  # This method is not covered by Rspec because it ends the current Ruby process.
   def exec_cmd(*args)
     Kernel::exec({'PATH' => ENV['PATH'], 'GL_ID' => ENV['GL_ID']}, *args, unsetenv_others: true)
   end