Add the Meaning of PHPUnit Output "W" to Docs

[morehawes]

Issue Description

When running PHPUnit tests, the letter "W" in yellow is output, but the meaning of this is not mentioned in the documentation.

URL of the Page with the Issue

Contribute with Testing > Automated Testing > PHP: PHPUnit

Section of Page with the issue

Running the Test Suite > What each symbol means

Why is this a problem?

When I ran the tests "W" was printed 86 times, but the documentation does not give the meaning of this.

Suggested Fix

Add "W" to the list of "What each symbol means". According to the PHPUnit docs:

W = Printed when the test triggered a warning

[github-actions[bot]]

Heads up @WordPress/docs-issues-coordinators, we have a new issue open. Time to use 'em labels.