Update guide with instructions to disable spinner

[jacobtolar]

TOX_PARALLEL_NO_SPINNER is mentioned in the command-line help, but can't be found anywhere obvious in the documentation besides the changelog.

Contribution checklist:

• wrote descriptive pull request text
• added/updated test(s) N/A
• updated/extended the documentation
• added relevant issue keyword
  in message body N/A
• added news fragment in changelog folder
  • fragment name: <issue number>.<type>.rst for example (588.bugfix.rst)
  • <type> is must be one of bugfix, feature, deprecation,breaking, doc, misc
  • if PR has no issue: consider creating one first or change it to the PR number after creating the PR
  • "sign" fragment with "by :user:<your username>"
  • please use full sentences with correct case and punctuation, for example: "Fix issue with non-ascii contents in doctest text files - by :user:superuser."
  • also see examples
• added yourself to CONTRIBUTORS (preserving alphabetical order)

I'm not sure the last two tasks add much value for a one-line documentation change, but I can update the changelog and contributors file if requested.