Fixed the retriever builder comments and error message

Author: mridula-s109

x-pack/plugin/search-business-rules/src/main/java/org/elasticsearch/xpack/searchbusinessrules/retriever/PinnedRetrieverBuilder.java

@@ -157,14 +157,13 @@ public int rankWindowSize() {
 
     /**
      * Creates a PinnedQueryBuilder with the appropriate pinned documents.
-     * Prioritizes docs over ids if both are present.
      *
      * @param baseQuery the base query to pin documents to
      * @return a PinnedQueryBuilder
      * @throws IllegalArgumentException if baseQuery is null
      */
     private QueryBuilder createPinnedQuery(QueryBuilder baseQuery) {
-        Objects.requireNonNull(baseQuery, "Underlying query cannot be null for pinned retriever");
+        Objects.requireNonNull(baseQuery, "pinned retriever requires retriever with associated query");
 
         if (docs != null && docs.isEmpty() == false) {
             return new PinnedQueryBuilder(baseQuery, docs.toArray(new SpecifiedDocument[0]));
@@ -177,7 +176,7 @@ protected SearchSourceBuilder finalizeSourceBuilder(SearchSourceBuilder source)
         validateSort(source);
         QueryBuilder underlyingQuery = source.query();
         if (underlyingQuery == null) {
-            throw new IllegalArgumentException("[underlying query] must not be null for pinned retriever");
+            throw new IllegalArgumentException("pinned retriever requires retriever with associated query");
         }
         source.query(createPinnedQuery(underlyingQuery));
         return source;

x-pack/plugin/search-business-rules/src/yamlRestTest/resources/rest-api-spec/test/search-business-rules/10_pinned_retriever.yml

@@ -379,7 +379,7 @@ setup:
 ---
 "pinned retriever error case - null inner query":
   - do:
-      catch: /illegal_argument_exception.+reason=\[underlying query\] must not be null for pinned retriever/
+      catch: /illegal_argument_exception.+reason=pinned retriever requires retriever with associated query/
       search:
         index: test-index1
         body: