doc: Improve Kconfig interface description

Pressing Y/N to configure boolean configuration symbols probably won't
be supported in the upcoming Python menuconfig implementation, and Space
is much smoother anyway. '?' is smoother than tabbing to '< Help >' too.

Also remove this part, which I couldn't make sense of:

    When a non-default entry is selected for options that are
    non-numerical, an asterisk :kbd:`*` appears between the square
    brackets in the display. There is nothing added added the display
    when you select the option's default.

[*] just means the current symbol value is 'y'.

Signed-off-by: Ulf Magnusson <ulfalizer@gmail.com>