Add explanation of BytesRef encodings

Author: parkertimmins

server/src/main/java/org/elasticsearch/index/mapper/BlockDocValuesReader.java

@@ -917,6 +917,9 @@ public int docId() {
         abstract void read(int docId, BytesRefBuilder builder) throws IOException;
     }
 
+    /**
+     * Read BinaryDocValues encoded by {@link BinaryFieldMapper.CustomBinaryDocValuesField}
+     */
     static class BytesRefsFromBinary extends AbstractBytesRefsFromBinary {
         private final ByteArrayStreamInput in = new ByteArrayStreamInput();
         private final BytesRef scratch = new BytesRef();
@@ -959,6 +962,10 @@ public String toString() {
         }
     }
 
+    /**
+     * Read BinaryDocValues with no additional structure in the BytesRefs.
+     * Each BytesRef from the doc values maps directly to a value in the block loader.
+     */
     public static class BytesRefsFromSimpleBinary extends AbstractBytesRefsFromBinary {
         public BytesRefsFromSimpleBinary(BinaryDocValues docValues) {
             super(docValues);