Update capability description

Author: alex-spies

x-pack/plugin/esql/src/main/java/org/elasticsearch/xpack/esql/action/EsqlCapabilities.java

@@ -950,7 +950,9 @@ public enum Cap {
         FIX_REPLACE_MISSING_FIELD_WITH_NULL_DUPLICATE_NAME_ID_IN_LAYOUT,
 
         /**
-         * Fix https://github.com/elastic/elasticsearch/issues/125850
+         * When creating constant null blocks in {@link org.elasticsearch.compute.lucene.ValuesSourceReaderOperator}, we also handed off
+         * the ownership of that block - but didn't account for the fact that the caller might close it, leading to double releases
+         * in some union type queries. C.f. https://github.com/elastic/elasticsearch/issues/125850
          */
         FIX_DOUBLY_RELEASED_NULL_BLOCKS_IN_VALUESOURCEREADER;