[Docs Bug] Explain severity values

[molant]

We need to document the severity changes somewhere in the docs and probably link from the browser extension and online scanner.

• Error - Will definitely not work as intended in at least one of the target browsers
• Warning - Minor impact or may not cause problems in practice (even if technically "wrong")
• Hint - Fine for now, but could cause problems in the future if something changes
• Information - Additional context or FYI, possibly to help with other parts of a feature

When configuring hints via .hintrc, the value Default can also be used to let webhint choose the appropriate level (typically when setting a configuration for some other aspect of a hint but wanting to keep the default severity).

[hxlnt]

@Reezaali will be working on this one next sprint.

[hxlnt]

I think we fixed this and forgot to link the PR to this issue See https://webhint.io/docs/user-guide/concepts/hints/. @antross @molant, feel free to reopen if this is in error.