kconfig: Clarify menu and 'if' dependency propagation

It is not obvious that the last two cases refer to menus and ifs,
respectively, in the conditional that sets 'parentdep'.

Automatic submenu creation is done later, so the parent can't be a
symbol here.

No functional changes. Only comments added.

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