take away TODO and replace with documentation, see #1621

js/input/Hotkey.ts

@@ -39,6 +39,8 @@ type SelfOptions = {
   // It is a PhET-specific concept that allows other non-modifier keys to be used as modifiers.
   // The standard modifier keys (ctrl/alt/meta/shift) are automatically handled by the hotkey system, but this can
   // expand the set of modifier keys that can be used.
+  // When a modifier key is added, pressing it will prevent any other Hotkeys from becoming active. This is how the
+  // typical modifier keys behave and so that is kept consistent for PhET-specific modifier keys.
   modifierKeys?: EnglishKey[];
 
   // A set of modifier keys that can be down and the hotkey will still fire. Essentially ignoring the modifier

js/input/hotkeyManager.ts

@@ -48,8 +48,8 @@ class HotkeyManager {
   private englishKeysDown: Set<EnglishKey> = new Set<EnglishKey>();
 
   // The current set of modifier keys (pressed or not) based on current enabled hotkeys
-  // TODO: Should we actually only have a set of modifier keys PER main key? https://github.com/phetsims/scenery/issues/1621
-  // TODO: e.g. should "b+x" (b being pressed) prevent "y"? https://github.com/phetsims/scenery/issues/1621
+  // NOTE: Pressed modifier keys will prevent any other Hotkeys from becoming active. For example if you have a hotkey
+  // with 'b+x', pressing 'b' will prevent any other hotkeys from becoming active.
   private modifierKeys: EnglishKey[] = [];
 
   // Hotkeys that are actively pressed
@@ -278,6 +278,7 @@ class HotkeyManager {
     this.activeHotkeys.delete( hotkey );
   }
 }
+
 scenery.register( 'HotkeyManager', HotkeyManager );
 
 const hotkeyManager = new HotkeyManager();