Do not warn on potentially unsafe HTML comments when unsafe=false

[bep]

We will still not render these comments, so from a safety perspective this is the same, but HTML comments are very common also inside Markdown and too useful to throw away.

Updates #13278

[jmooring]

The blocks work great, but these emit warnings:

This is a <!--hidden--> word.

This is a <!-- hidden--> word.

This is a <!-- hidden --> word.

This is a <!-- 
hidden --> word.

This is a <!-- 
hidden
--> word.

[bep]

Thanks, I forgot that we had 2 variants, pushed a fixed in 22145b5

[jmooring]

This works great. When released we should immediately deprecate the comment shortcode (emit warning). After a year or so throw an error. And probably keep that one-line shortcode for a few years.