Hello,
Just as an FYI, Kconfiglib 2 (https://github.com/ulfalizer/kconfiglib) can generate nice Kconfig docs. See http://docs.zephyrproject.org/reference/kconfig/index.html for example output (warning: heavy page, lots of symbols).
The script that generates the RST output for those docs is at https://github.com/zephyrproject-rtos/zephyr/blob/master/doc/scripts/genrest.... It deals well with symbols defined in multiple locations as well (plenty of those in Zephyr too), listing each definition with the correct properties.
I just added a patch to Kconfiglib that reduces the parsing time for the U-Boot Kconfigs from about four seconds to about half a second as well (Core i7 2600k). Those symbols defined in a million locations triggered some redundant work earlier. :)
Cheers, Ulf