Update mongo resync documentation
Update mongo resync documentation

- Don't sync more than one secondary at once
- Resyncing a primary isn't valid and makes no sense, we shouldn't
  document it
- Make fab script documentation consistent with elsewhere in the docs

alphagov/govuk-developer-docs,alphagov/govuk-developer-docs,alphagov/govuk-developer-docs,alphagov/govuk-developer-docs