Yes, the autodoc currently just say "Sets including the character ']' must list this first too.", which is a bit terse. The Solaris man-page says:
If the conversion specification begins with [] or [^], the right square bracket is included in the scanlist and the next right square bracket is the matching right square bracket that ends the conversion specification; otherwise the first right square bracket is the one that ends the conversion specification.
which should be clear enough.