kconfig: Document automatic submenu creation code

It's tricky to figure out what it does (and how) without staring at the
code for a long time. Document it to make it more transparent.

No functional changes. Only comments added.

Signed-off-by: Ulf Magnusson <ulfalizer@gmail.com>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>