Add DevcontainerDown to help

**Describe the bug**
Currently in the help for the plugin the is no description of `DevcontainerDown`

**To Reproduce**
Steps to reproduce the behavior:
1. Go to `:help devcontainer-cli`
2. No description of `DevcontainerDown`

**Expected behavior**
An entry for `DevcontainerDown` should be in the help.

**Desktop (please complete the following information):**
 - OS: Linux
