Usage of [!nonportable] unclear

**Describe the bug**
The documentation (docs/test-cases-and-sections.md) describes a list of "special tags" with their effects on the test run. However, for `[!nonportable]` that description is missing. It is not clear if this tag has any effect or is just for documentary purposes.

**Expected behavior**
Make `[!nonportable]` have an effect on the test run and/or describe its effect in the documentation.