docs: clarify compound extension signature naming for bools

[gforsyth]

537 clarified that compound extension signatures are now required.

I noticed that the table specifying how to type those signatures didn't include booleans -- I think it's probably bool but either way it should be included.

Also in this PR, a quick indentation fix so that the Note: admonitions render correctly (there needs to be blank line and then an indented block)

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[gforsyth]

Current website

Fixed admonition