A `<replaceable>` is a placeholder that a user is instructed to replace with a value of their own, like `<replaceable>prefix</replacable>/bin/foo`. In the standard Docbook toolchain, this typically appears emphasized, and no other adornement. But a `<replaceable>` is nearly always in a code element, where emphasis won't work. So we do the same thing as for `<optional>`: decorate the content with brackets.