Minor fixes for apidocs and comments (#11249)

* improve comments for api.getUserChallenges

* clarify members parameter when creating test guilds

I was uncertain about this parameter and read the code to clarify it.
Reading a comment is easier. :)

* add "Custom Day Start time" to api route's title as it's the common name

website/server/controllers/api-v3/challenges.js

@@ -425,11 +425,11 @@ api.getUserChallenges = {
 
 /**
  * @api {get} /api/v3/challenges/groups/:groupId Get challenges for a group
- * @apiDescription Get challenges that the user is a member, public challenges and the ones from the user's groups.
+ * @apiDescription Get challenges hosted in the specified group.
  * @apiName GetGroupChallenges
  * @apiGroup Challenge
  *
- * @apiParam (Path) {UUID} groupId The group _id
+ * @apiParam (Path) {UUID} groupId The group id ('party' for the user party and 'habitrpg' for tavern are accepted)
  *
  * @apiSuccess {Array} data An array of challenges sorted with official challenges first, followed by the challenges in order from newest to oldest
  *
@@ -462,7 +462,7 @@ api.getGroupChallenges = {
 
     const challenges = await Challenge.find({ group: groupId })
       .sort('-createdAt')
-      // .populate('leader', nameFields) // Only populate the leader as the group is implicit
+      // .populate('leader', nameFields) // Only populate the leader as the group is implicit // see below why we're not using populate
       .exec();
 
     let resChals = challenges.map(challenge => challenge.toJSON());