kconfig: Document the 'symbol' struct

Visibility and choices in particular might be a bit tricky to figure
out.

Also fix existing comment to point out that P_MENU is also used for
menus.

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