added comment, why we fail here.

core/src/main/java/org/elasticsearch/index/percolator/PercolatorFieldMapper.java

@@ -177,6 +177,9 @@ public class PercolatorFieldMapper extends FieldMapper {
             if (fieldMapper instanceof PercolatorFieldMapper) {
                 PercolatorFieldType fieldType = (PercolatorFieldType) fieldMapper.fieldType();
                 if (context.doc().getField(fieldType.getQueryBuilderFieldName()) != null) {
+                    // If a percolator query has been defined in an array object then multiple percolator queries
+                    // could be provided. In order to prevent this we fail if we try to parse more than one query
+                    // for the current document.
                     throw new IllegalArgumentException("a document can only contain one percolator query");
                 }
             }