Add helpful config comments

I just spent half an hour debugging why some setting didn't work for me.
It was because I was putting it in Chat instead of Keys.Chat. This is
very confusing when you're making your own partial config override file.
I've added comments to the classes with identical names to hopefully
make this less confusing.

src/config/settings.py

@@ -288,7 +288,7 @@ class Keys:
     # Quit Mirage
     quit = []
 
-    class Scrolling:
+    class Scrolling: # Keys.Scrolling
         # Pages and chat timeline scrolling
         up        = ["Alt+Up", "Alt+K"]
         down      = ["Alt+Down", "Alt+J"]
@@ -322,7 +322,7 @@ class Keys:
         previous = ["Alt+Shift+N"]
         next     = ["Alt+N"]
 
-        class AtIndex:
+        class AtIndex: # Accounts.AtIndex
             # Switch to the first room of the account number X in the list.
             # Each property is a list of keybinds for the account number X.
             # Numbers beyond the default ones can be added.
@@ -373,7 +373,7 @@ class Keys:
         oldest_highlight = ["Ctrl+Shift+M"]
         latest_highlight = ["Ctrl+M"]
 
-        class AtIndex:
+        class AtIndex: # Rooms.AtIndex
             # Switch to room number X in the current account.
             # Each property is a list of keybinds for the room number X:
             # Numbers beyond the default ones can be added.
@@ -400,7 +400,7 @@ class Keys:
             # which account should be targeted as "<userId> <roomId>":
             "@account:example.org !roomID:example.org" = []
 
-    class Chat:
+    class Chat: # Keys.Chat
         # Keybinds specific to the current chat page.
 
         # Focus the right room pane. If the pane is currently showing the