docs: described exit codes in npm-audit docs

[emilis-tm]

Background

It was very easy to just append npm audit to my npm test script, because it returns a correct exit status when vulnerabilities are found.

I think the doc page (https://docs.npmjs.com/cli/audit) should tell readers about the exit status. Had to run my own mini-experiment to see if it works the way I expected.

Change

• I added a sentence about the exit status to the description block.
• I added a section with further description to the page:
  • below CONTENT SUBMITTED, because of the order in the DESCRIPTION paragraph,
  • felt the need to mention audit-level configuration setting.

[zkat]

This looks good! I'd appreciate it if you could rebase this onto latest. You seem to have slurped up a stray commit. I'm otherwise good with merging this 👍

[zkat]

Merged this one by hand. :) Thank you!