Add javadocs for Lucene 7 codec classes (#117819)

Author: javanna

x-pack/plugin/old-lucene-versions/src/main/java/org/elasticsearch/xpack/lucene/bwc/codecs/lucene70/BWCLucene70Codec.java

@@ -25,6 +25,12 @@
 import org.elasticsearch.xpack.lucene.bwc.codecs.BWCCodec;
 import org.elasticsearch.xpack.lucene.bwc.codecs.lucene60.Lucene60MetadataOnlyPointsFormat;
 
+/**
+ * Implements the Lucene 7.0 index format. Loaded via SPI for indices created/written with Lucene 7.x (Elasticsearch 6.x) mounted
+ * as archive indices first in Elasticsearch 8.x. Lucene 9.12 retained Lucene70Codec in its classpath which required overriding the
+ * codec name and version in the segment infos. This codec is still needed after upgrading to Elasticsearch 9.x because its codec
+ * name has been written to disk.
+ */
 public class BWCLucene70Codec extends BWCCodec {
 
     private final FieldInfosFormat fieldInfosFormat = wrap(new Lucene60FieldInfosFormat());
@@ -46,6 +52,8 @@ public PostingsFormat getPostingsFormatForField(String field) {
         }
     };
 
+    // Needed for SPI loading
+    @SuppressWarnings("unused")
     public BWCLucene70Codec() {
         this("BWCLucene70Codec");
     }

x-pack/plugin/old-lucene-versions/src/main/java/org/elasticsearch/xpack/lucene/bwc/codecs/lucene70/Lucene70Codec.java

@@ -7,6 +7,14 @@
 
 package org.elasticsearch.xpack.lucene.bwc.codecs.lucene70;
 
+/**
+ * Implements the Lucene 7.0 index format. Will be loaded via SPI for indices created/written with Lucene 7.x (Elasticsearch 6.x) mounted
+ * as archive indices in Elasticsearch 9.x. Note that for indices with same version mounted first as archive indices in Elasticsearch 8.x,
+ * {@link BWCLucene70Codec} will be instead used which provides the same functionality, only registered with a different name.
+ *
+ * @deprecated Only for 7.0 back compat
+ */
+@Deprecated
 public class Lucene70Codec extends BWCLucene70Codec {
 
     public Lucene70Codec() {