Fixes for PR 2397.

- GroupManager now validates core groups in the constructor.
- EnsureCoreGroupsPresent -> AssertCoreGroupsPresent.
- Fix indentiation in some places.
- Clarify the intent of this PR in CHANGELOG.md.

TShockAPI/Utils.cs

@@ -216,11 +216,12 @@ namespace TShockAPI
 		/// Asserts that the group reference can be safely assigned to the player object.
 		/// <para>If this assertion fails, and <paramref name="kick"/> is true, the player is disconnected. If <paramref name="kick"/> is false, the player will receive an error message.</para>
 		/// </summary>
-		/// <param name="player"></param>
-		/// <param name="group"></param>
+		/// <param name="player">The player in question</param>
+		/// <param name="group">The group we want to assign them</param>
+		/// <param name="kick">Whether or not failing this check disconnects the player.</param>
 		/// <returns></returns>
 		public bool AssertGroupValid(TSPlayer player, Group group, bool kick)
-        {
+		{
 			if (group == null)
 			{
 				if (kick)