Merge pull request #631 from replaceitem/scoreboard-criterion-mod

Added ability to change criterion with scoreboard_add Fixes #624

docs/scarpet/api/Scoreboard.md

@@ -10,8 +10,9 @@ With specified `objective` and
  
 ### `scoreboard_add(objective, criterion?)`
 
-Adds a new objective to scoreboard. If `criterion` is not specified, assumes `'dummy'`. Returns `false` if objective 
-already existed, `true` otherwise.
+Adds a new objective to scoreboard. If `criterion` is not specified, assumes `'dummy'`.
+If the objective already exists, changes the criterion of that objective and returns `false`. If the criterion was not specified but the objective already exists, returns the current criterion.
+If the objective was added, returns `true`. If nothing is affected, returns `null`
 
 <pre>
 scoreboard_add('counter')
@@ -162,4 +163,3 @@ for(bossbar(),bossbar(_,'remove'))  -> remove all bossbars
 
 
 
-