Remove mention of reverse in docs and add to migration doc

docs/reference/migration/migrate_5_0/search.asciidoc

@@ -141,3 +141,8 @@ The term vectors APIs no longer persist unmapped fields in the mappings.
 
 The `dfs` parameter to the term vectors API has been removed completely. Term
 vectors don't support distributed document frequencies anymore.
+
+==== Sort
+
+The `reverse` parameter has been removed, in favour of explicitly
+specifying the sort order with the `order` option.

docs/reference/search/request/sort.asciidoc

@@ -346,7 +346,7 @@ When sorting on a field, scores are not computed. By setting
 {
     "track_scores": true,
     "sort" : [
-        { "post_date" : {"reverse" : true} },
+        { "post_date" : {"order" : "desc"} },
         { "name" : "desc" },
         { "age" : "desc" }
     ],