Add translation notes.

[ci skip]

(cherry picked from commit 46739fc)

data/gui/window/preferences/02_hotkeys.cfg

@@ -131,6 +131,7 @@
 								id = "sort_2"
 								definition = "listbox_header"
 								linked_group = "hotkeys_col_type"
+								# po: Translate G as the initial letter for Game
 								label = _ "game_hotkeys^G"
 								tooltip = _ "Available in game"
 							[/toggle_button]
@@ -143,6 +144,7 @@
 								id = "sort_3"
 								definition = "listbox_header"
 								linked_group = "hotkeys_col_type"
+								# po: Translate E as the initial letter for Editor
 								label = _ "editor_hotkeys^E"
 								tooltip = _ "Available in editor"
 							[/toggle_button]
@@ -155,6 +157,7 @@
 								id = "sort_4"
 								definition = "listbox_header"
 								linked_group = "hotkeys_col_type"
+								# po: Translate T as the initial letter for Title Screen
 								label = _ "titlescreen_hotkeys^T"
 								tooltip = _ "Available at main menu"
 							[/toggle_button]