Add docs, small fixes

Author: carlosdelest

server/src/main/java/org/elasticsearch/index/engine/InternalEngine.java

@@ -2850,7 +2850,7 @@ public synchronized void beforeMerge(OnGoingMerge merge) {
         @Override
         protected long estimateMergeMemory(MergePolicy.OneMerge merge) {
             try (Searcher searcher = acquireSearcher("merge_memory_estimation", SearcherScope.INTERNAL)) {
-                return SegmentMergeMemoryEstimator.estimateMergeMemory(merge, searcher.getIndexReader());
+                return MergeMemoryEstimator.estimateMergeMemory(merge, searcher.getIndexReader());
             } catch (AlreadyClosedException e) {
                 failOnTragicEvent(e);
                 return 0L;

server/src/main/java/org/elasticsearch/index/engine/SegmentMergeMemoryEstimator.java renamed to server/src/main/java/org/elasticsearch/index/engine/MergeMemoryEstimator.java

@@ -23,8 +23,17 @@
 
 import java.util.List;
 
-public class SegmentMergeMemoryEstimator {
+/**
+ * Provides an estimation of the memory needed to merge segments.
+ *
+ * This class is a temporary solution until we have a better way to estimate the memory needed for merges in Lucene.
+ * We can work iteratively in providing estimations for different types of fields and vector encodings.
+ */
+public class MergeMemoryEstimator {
 
+    /**
+     * Estimates the memory, in bytes, needed to merge the segments of the given merge.
+     */
     public static long estimateMergeMemory(MergePolicy.OneMerge merge, IndexReader indexReader) {
         long memoryNeeded = 0;
         for (SegmentCommitInfo mergedSegment : merge.segments) {