update api.txt

src/main/webapp/doc/api.txt

@@ -12,6 +12,14 @@ save (json required) {
     return saved dict json with id
 }
 update (id required, json required) {
+    json {
+        filter: {k:v...},
+        update: {
+            $set: {k,v...},
+            $unset: [k1,k2...],
+            $rename: {k,newK...}
+        }
+    }
    return {result : true or false}
 }
 delete (id required) {