The suggested search phrases field is for something else entirely: https://goo.gl/2n2bNN
Basically with that option you can control those words for each single post individually.
If you want to add additional words, I suggest creating a custom field. For example you can make a custom field called search_keywords then enter the keywords separated by space instead of comma: http://i.imgur.com/rOlRqSu.png
Then, on the General Options -> Sources panel you can select this custom field as a source as well: http://i.imgur.com/AVO1UjI.png