[whatwg/url] Include invalid outputs in the table with valid versus invalid parser inputs (Issue #856)

alwinb created an issue (whatwg/url#856)

### What is the issue with the URL Standard?

At the start of section **4. URLs**, there is a [table](https://url.spec.whatwg.org/#example-url-parsing) that illustrates various combinations of input– and base URLs. The table expresses if the _combination of the input and the base_ is considered valid; and what the resulting output is. 

I propose to include examples that produce an **invalid output URL** in addition to those that cause Failure, and indicate this clearly in the table. 

Maybe this can be done by adding another column with ✅ and ❌ markers for the output.

I think it would be useful to also call out the following phrase just above the table into a separate block:

> The output of the URL serializer is not always a valid URL string.

Either in one of the green Note blocks, or more appropriately, 
in the red with the red outline.

(As a bonus, we can include footnotes of some sort that link the the various validation issues, though it might be tricky to get a nice layout).

Motivation:

The differences between valid, invalid and failure, are often a source of confusion. Especially the fact that the parser can **Produce invalid URLs** can be counter intuitive. 

This issue was taken out of the discussion in #379, cc @domenic

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