Documentation for score_mode

Closes #4742

docs/reference/search/request/rescore.asciidoc

@@ -28,10 +28,10 @@ number of docs which will be examined on each shard can be controlled by
 the `window_size` parameter, which defaults to
 <<search-request-from-size,`from` and `size`>>.
 
-The scores from the original query and the rescore query are combined
-linearly to produce the final `_score` for each document. The relative
-importance of the original query and of the rescore query can be
-controlled with the `query_weight` and `rescore_query_weight`
+By default the scores from the original query and the rescore query are
+combined linearly to produce the final `_score` for each document. The
+relative importance of the original query and of the rescore query can
+be controlled with the `query_weight` and `rescore_query_weight`
 respectively. Both default to `1`.
 
 For example:
@@ -67,3 +67,15 @@ curl -s -XPOST 'localhost:9200/_search' -d '{
 }
 '
 --------------------------------------------------
+
+The way the scores are combined can be controled with the `score_mode`:
+[cols="<,<",options="header",]
+|=======================================================================
+|Score Mode |Description
+|`total`    |Add the original score and the rescore query score.  The default.
+|`multiply` |Multiply the original score by the rescore query score.  Useful
+for <<query-dsl-function-score-query,`function query`>> rescores.
+|`avg`      |Average the original score and the rescore query score.
+|`max`      |Take the max of original score and the rescore query score.
+|`min`      |Take the min of the original score and the rescore query score.
+|=======================================================================