NMS-7267: Unclear-module-description-admin

- Improved module description for guide-doc
- Improved module description for guide-user
- Improved module description for guide-admin

JIRA: http://issues.opennms.org/browse/NMS-7267

Todo:
- [ ] Typo and Grammar
- [ ] Formatting and conventions
- [ ] Content
- [ ] Close JIRA issue