Improve docs on Kfamily

[gvegayon]

Add to the docs of the Korean family:

• The survey has information about the current method of family planning, which could be none. The current one is on 1974.
• But then, it also includes a set of twelve questions asking about their historical use, in other words, what they used in the past, before the current (1964 - 1973).
• This can be used to generate an adoption/disadoption dataset.