Swift: Work around some QHelp rendering issues.

[geoffw0]

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A few of the Swift .qhelp files have minor rendering issues now that we label the code snippets as Swift (and have syntax highlighting). The issue is believed to be caused by an external component, so for now lets just work around it by rendering problematic snippets without syntax highlighting.

~(I'm not 100% sure I have the syntax correct for this, so lets wait for the previews to be sure)~ all good now.