Add document highlight function

I added a document about highlight_html() and highlight_full().
Please refer the following preview.

http://createfield.com/en/reference/functions/highlight_full.html
http://createfield.com/en/reference/functions/highlight_html.html
http://createfield.com/ja/reference/functions/highlight_full.html
http://createfield.com/ja/reference/functions/highlight_html.html