Fix wording, formatting and link in offline upgrade/downgrade

Changes:
* reformat the last step of the offline upgrade section so that
  instructions and their descriptions are more clear
* update `reindex` command link to version `3.10.0`
* replace `v3.7/v3.8` with `v3.9` in downgrade section
* rephrase the step 3 so that it doesn't refer to `changes` as all
  indices will be affected
* reformat downgrade section so that code snippet is part of step 3
  and step 4 correctly numbered

Change-Id: I8376197e56ec5b6076c8d0bf9cbfa913864b54c2
1 file changed