newlines within bold/italics/strikethrough in docs don't work

[iamevn]

doc = {{
  **This doc is bold and has
  a linebreak.**
}}

causes UCM to display an error:

unexpected "<newline>  a linebreak.*"
expecting "**", "@eval", "@foldedSource", "@inlineSignature", "@signature", "@signatures", "@source", code (examples: ''**unformatted**'', '''_words_'''), hyperlink (example: [link name](https://destination.com)), inline code (examples: ``List.map f xs``, ``[1] :+ 2``), link (examples: {type List}, {Nat.+}), or transclusion (examples: {{ doc2 }}, {{ sepBy s [doc1, doc2] }})

If you add a doc with a long line and then edit it, the version in your scratch file will have line breaks added and those may split bold/italics/strikethrough wrappers and trigger this.