Add `--watchAll=false` to coverage docs

Created by: ianjmacintosh

Is your proposal related to a problem?

Yes, when I follow the instructions on https://create-react-app.dev/docs/running-tests/#coverage-reporting to run coverage reporting, I get an empty report. Others have reported this in #6888 (closed) #7838 #9322 (closed) #9876.

Describe the solution you'd like

Until this changes, I propose updating https://github.com/facebook/create-react-app/blame/master/docusaurus/docs/running-tests.md#L179 to prescribe npm test -- --coverage --watchAll=false

Describe alternatives you've considered

Ideally this behavior would be changed by the Jest team, but https://github.com/facebook/jest/issues/7331 doesn't seem to have much activity. Maybe there's another issue that's tracking it.