doc: extensions: kconfig: handle promptless choices showing as parents

Promptless choices can show up as parents when, e.g., people define
choices in multiple locations, including modules. Render them using the
built-in Kconfig expression to string formatter, so that they show up as
'<choice (...)>'.

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>