Add documentation

server/src/main/java/org/elasticsearch/index/shard/IndexShard.java

@@ -3615,6 +3615,12 @@ public final void sync(Translog.Location location, Consumer<Exception> syncListe
         getEngine().asyncEnsureTranslogSynced(location, syncListener);
     }
 
+    /**
+     * This method provides the same behavior as #sync but for persisting the global checkpoint. It will initiate a sync
+     * if the request global checkpoint is greater than the currently persisted global checkpoint. However, sam as #sync it
+     * will not ensure that the request global checkpoint is available to be synced. It is the caller's duty to only call this
+     * method with a valid processed global checkpoint that is available to sync.
+     */
     public final void syncGlobalCheckpoint(long globalCheckpoint, Consumer<Exception> syncListener) {
         verifyNotClosed();
         getEngine().asyncEnsureGlobalCheckpointSynced(globalCheckpoint, syncListener);