[plt-scheme] _id in Scribble

If I want to document a symbol with a leading underscore, Scribble drops 
the underscore. IIUC, this is what the @schemeblock docs are talking 
about: `_id' is treated as `id' but with `schemevarfont'. But the docs 
are a little confusing about exactly when this behavior should apply.

Specifically, when I write

     @scheme['_Bool]

I get the ordinary datum font (not schemevarfont) but with the 
underscore dropped. What's the recommended way to disable the processing 
of the underscore?

Thanks,
Dave