Add documentation

SessionMessagingKit/Sending & Receiving/MessageReceiver+Handling.swift

@@ -484,9 +484,17 @@ extension MessageReceiver {
             let newGroupModel = TSGroupModel(title: group.groupName, memberIds: [String](members), image: nil, groupId: groupID, groupType: .closedGroup, adminIds: group.groupAdminIds)
             thread.setGroupModel(newGroupModel, with: transaction)
             // Send the latest encryption key pair to the added members if the current user is the admin of the group
-            // FIXME: Is this still needed? I believe this was to fix a race condition, where someone might add a
-            // member to a group when they don't have the latest key pair. The admin would then be able to send the
-            // added user the latest key pair later. Not sure if that race condition still exists though.
+            //
+            // This fixes a race condition where:
+            // • A member removes another member.
+            // • A member adds someone to the group and sends them the latest group key pair.
+            // • The admin is offline during all of this.
+            // • When the admin comes back online they see the member removed message and generate + distribute a new key pair,
+            //   but they don't know about the added member yet.
+            // • Now they see the member added message.
+            //
+            // Without the code below, the added member(s) would never get the key pair that was generated by the admin when they saw
+            // the member removed message.
             let isCurrentUserAdmin = group.groupAdminIds.contains(getUserHexEncodedPublicKey())
             if isCurrentUserAdmin {
                 for member in membersAsData.map({ $0.toHexString() }) {