fix docs; add backport 8.x index version

Author: markjhoy

docs/reference/elasticsearch/mapping-reference/sparse-vector.md

@@ -59,7 +59,7 @@ The following parameters are accepted by `sparse_vector` fields:
     * Use [synthetic `_source`](/reference/elasticsearch/mapping-reference/mapping-source-field.md#synthetic-source).
 
 index_options
-:    (Optional, object) You can set index options for your  `sparse_vector` field to determine if you should prune tokens, and the parameter configurations for the token pruning. If the pruning options are not set for your `sparse_query` vector on the field, Elasticsearch will use the defaults if set here for the field. The available options for the index options are:
+:   (Optional, object) You can set index options for your  `sparse_vector` field to determine if you should prune tokens, and the parameter configurations for the token pruning. If pruning options are not set in your `sparse_query` vector, Elasticsearch will use the default options configured for the field, if any. The available options for the index options are:
 
 Parameters for `index_options` are:
 

server/src/main/java/org/elasticsearch/index/IndexVersions.java

@@ -140,6 +140,7 @@ private static Version parseUnchecked(String version) {
     public static final IndexVersion ADD_RESCORE_PARAMS_TO_QUANTIZED_VECTORS_BACKPORT_8_X = def(8_528_0_00, Version.LUCENE_9_12_1);
     public static final IndexVersion RESCORE_PARAMS_ALLOW_ZERO_TO_QUANTIZED_VECTORS_BACKPORT_8_X = def(8_529_0_00, Version.LUCENE_9_12_1);
     public static final IndexVersion DEFAULT_OVERSAMPLE_VALUE_FOR_BBQ_BACKPORT_8_X = def(8_530_0_00, Version.LUCENE_9_12_1);
+    public static final IndexVersion SPARSE_VECTOR_PRUNING_INDEX_OPTIONS_SUPPORT_BACKPORT_8_X = def(8_531_0_00, Version.LUCENE_9_12_1);
     public static final IndexVersion UPGRADE_TO_LUCENE_10_0_0 = def(9_000_0_00, Version.LUCENE_10_0_0);
     public static final IndexVersion LOGSDB_DEFAULT_IGNORE_DYNAMIC_BEYOND_LIMIT = def(9_001_0_00, Version.LUCENE_10_0_0);
     public static final IndexVersion TIME_BASED_K_ORDERED_DOC_ID = def(9_002_0_00, Version.LUCENE_10_0_0);