address review comments

Author: javanna

server/src/main/java/org/elasticsearch/common/io/stream/StreamOutput.java

@@ -798,10 +798,19 @@ public <T> void writeOptionalArray(final Writer<T> writer, final @Nullable T[] a
         }
     }
 
+    /**
+     * Writes the specified array of {@link Writeable}s. This method can be seen as
+     * writer version of {@link StreamInput#readArray(Writeable.Reader, IntFunction)}. The length of array encoded as a variable-length
+     * integer is first written to the stream, and then the elements of the array are written to the stream.
+     */
     public <T extends Writeable> void writeArray(T[] array) throws IOException {
         writeArray((out, value) -> value.writeTo(out), array);
     }
 
+    /**
+     * Same as {@link #writeArray(Writeable[])} but the provided array may be null. An additional boolean value is
+     * serialized to indicate whether the array was null or not.
+     */
     public <T extends Writeable> void writeOptionalArray(@Nullable T[] array) throws IOException {
         writeOptionalArray((out, value) -> value.writeTo(out), array);
     }

server/src/main/java/org/elasticsearch/common/lucene/Lucene.java

@@ -565,18 +565,11 @@ public static SortField readSortField(StreamInput in) throws IOException {
     }
 
     public static SortField[] readSortFields(StreamInput in) throws IOException {
-        SortField[] fields = new SortField[in.readVInt()];
-        for (int i = 0; i < fields.length; i++) {
-            fields[i] = readSortField(in);
-        }
-        return fields;
+        return in.readArray(Lucene::readSortField, SortField[]::new);
     }
 
     public static void writeSortFields(StreamOutput out, SortField[] sortFields) throws IOException {
-        out.writeVInt(sortFields.length);
-        for (SortField sortField : sortFields) {
-            writeSortField(out, sortField);
-        }
+        out.writeArray(Lucene::writeSortField, sortFields);
     }
 
     public static void writeSortType(StreamOutput out, SortField.Type sortType) throws IOException {