Fix downgrade documentation

[jovatn]

Description

Recent documentation additions on upgrade/downgrade (#1009) did not properly describe how to best perform a downgrade. In particular, it is best to store a startup config before upgrading, and use that when downgrading

No changelog updates needed

Checklist

Tick relevant boxes, this PR is-a or has-a:

• [ ] Bugfix
  • [ ] Regression tests
  • [ ] ChangeLog updates (for next release)
• [ ] Feature
  • [ ] YANG model change => revision updated?
  • [ ] Regression tests added?
  • [ ] ChangeLog updates (for next release)
  • [ ] Documentation added?
• [ ] Test changes
  • [ ] Checked in changed Readme.adoc (make test-spec)
  • [ ] Added new test to group Readme.adoc and yaml file
• [ ] Code style update (formatting, renaming)
• [ ] Refactoring (please detail in commit messages)
• [ ] Build related changes
• [X] Documentation content changes
  • [ ] ChangeLog updated (for major changes)
• [ ] Other (please describe):