Reduce confusion in add-highlighter docstring
Reduce confusion in add-highlighter docstring

Closes #2222

lenormf/kakoune,Somasis/kakoune,casimir/kakoune,jkonecny12/kakoune,lenormf/kakoune,alexherbo2/kakoune,mawww/kakoune,alexherbo2/kakoune,danr/kakoune,mawww/kakoune,mawww/kakoune,alexherbo2/kakoune,occivink/kakoune,jkonecny12/kakoune,casimir/kakoune,jjthrash/kakoune,danr/kakoune,occivink/kakoune,jjthrash/kakoune,danr/kakoune,lenormf/kakoune,casimir/kakoune,alexherbo2/kakoune,jkonecny12/kakoune,mawww/kakoune,occivink/kakoune,occivink/kakoune,jjthrash/kakoune,lenormf/kakoune,jjthrash/kakoune,Somasis/kakoune,danr/kakoune,jkonecny12/kakoune,Somasis/kakoune,Somasis/kakoune,casimir/kakoune