setSlot() + addSlot() + output inconsistency

Problem/Motivation

Two things:

1. $ce = CustomElement::create('MYTAG')
     ->setSlot('content', 'FIRST')
     ->addSlot('content', 'SECOND');
   

   sets slotNormalizationStyle NORMALIZE_AS_SINGLE_VALUE. (Specifically: a setSlot() call without $index parameter does this.)

2. Rendering $ce as HTML produces

   <mytag><div slot="content">FIRST</div><div slot="content">SECOND</div></mytag>
   

   But normalizing JSON produces

   [ 'element' => 'mytag', 'content' => 'FIRST']
   

   In other words, the CustomElement::$slotNormalizationStyles property is only used for JSON output, not HTML.

Remaining tasks

Work out what we want to do with this.

• Do we not care about the difference between outputs? Then document the fact that setSlotNormalizationStyle() does nothing for HTML. Otherwise fix?
• Is it a known fact that a setSlot*() call without $index parameter 'breaks' subsequent addSlot*() calls for the same slot name but only for JSON, not HTML? Then document this.
• ...

(CustomElement and set/addSlot() calls' documentation could use an overhaul/extension, apart from this. I'll later decide whether to do this in this same issue, or maybe file a followup, or maybe I will have already done that before this issue is addressed...)