According to https://www.elastic.co/guide/en/elasticsearch/reference/current/percolator.html

Also an index can only contain up to one percolator field mapping. Multiple percolator fields will be rejected by the put index and put mapping APIs.

@martijnvg pointed out that this is not an accurate statement, and it's actually easy to verify. The following PUT command works fine:

PUT my_test_index
{
  "mappings": {
    "doc": {
      "properties": {
        "query1": {
          "type": "percolator"
        },
        "query2": {
          "type": "percolator"
        }
      }
    }
  }
}

I suspect the comment can simply be removed.