Clarify support of Markdown in labels

[nichtich]

So called markdown strings are misleading and error-prone because internally they are parsed as markdown with micromark (unless option htmlLabels is disabled and this function breaks valid markdown parsing) but then:

• all Markdown features except bold and italics lead to an error
• soft line breaks are converted to hard line breaks (some Markdown dialects do this but not in CommonMark)
• final HTML is further processed to replace entity codes, fontawesome strings and maybe more (this may be defined as extension to Markdown)

Then why calling it markdown at all? It's very easy to accidently get an error message instead of a proper label when using "markdown strings" (e.g. "`-`"). Is there a reason to only allow bold and italics?