[doc] Add example of -k 'not test' in help text

[ghost]

Fix #1442

Thanks for submitting a PR, your contribution is really appreciated!

Here's a quick checklist that should be present in PRs:

• Add a new news fragment into the changelog folder
  • name it $issue_id.$type for example (588.bug)
  • if you don't have an issue_id change it to the pr id after creating the pr
  • ensure type is one of removal, feature, bugfix, vendor, doc or trivial
  • Make sure to use full sentences with correct case and punctuation, for example: "Fix issue with non-ascii contents in doctest text files."
• Target: for bugfix, vendor, doc or trivial fixes, target master; for removals or features target features;
• Make sure to include reasonable tests for your change if necessary

Unless your change is a trivial or a documentation fix (e.g., a typo or reword of a small section) please:

• Add yourself to AUTHORS;

[coveralls]

Coverage increased (+28.9%) to 92.232% when pulling 7093d8f on xuanluong:issue-1442-mention-not--k-help into 966391c on pytest-dev:master.

[nicoddemus]

Awesome @Xuanluong, really appreciate this.