[Docs] html: shallow renders full tree

[zbycz]

I was very confused, why shallow().html() returns the whole HTML tree. This is kind of unxepected, so this PR can help others undestand.

I found that its expected behavior here: #388 (comment)

btw, how does it really work? Does it call the same render method for both mount() and shallow()? Possibly even for render()? Thanks!

[ljharb]

.html() calls into render.

What is probably wanted most of the time is .debug().

[zbycz]

@ljharb Thanks for the clarification! I accepted your suggestion and added a sentence about internal workings and hint for debug.

I hope several commits are fine - github can squash em, right?

[ljharb]

Thanks!