Update out-of-date Sampling section of docs

[joseph-robertson]

Pull Request Description

In response to https://unmethours.com/question/96702/resstock-sampling-file/.

Checklist

Not all may apply:

• [ ] Tests (and test files) have been updated
• [x] Documentation has been updated
  • [ ] If related to resstock-estimation, checklist includes data dictionary, source report, options_lookup.
  • [ ] If changes to project_testing tsvs, checklist includes yml_precomputed, yml_precomputed_outdated, yml_precomputed_weight
• [ ] Changelog has been updated
• [ ] openstudio tasks.rb update_measures has been run
• [ ] No unexpected regression test changes on CI (checked comparison artifacts)