update some copy for the background opacity toggle

src/apprt/action.zig

@@ -115,7 +115,9 @@ pub const Action = union(Key) {
     /// Toggle the visibility of all Ghostty terminal windows.
     toggle_visibility,
 
-    /// Toggle the window background opacity. This currently only works on macOS.
+    /// Toggle the window background opacity. This only has an effect
+    /// if the window started as transparent (non-opaque), and toggles
+    /// it between fully opaque and the configured background opacity.
     toggle_background_opacity,
 
     /// Moves a tab by a relative offset.

src/input/command.zig

@@ -621,7 +621,7 @@ fn actionCommands(action: Action.Key) []const Command {
         .toggle_background_opacity => comptime &.{.{
             .action = .toggle_background_opacity,
             .title = "Toggle Background Opacity",
-            .description = "Toggle the window background between transparent and opaque.",
+            .description = "Toggle the background opacity of a window that started transparent.",
         }},
 
         .check_for_updates => comptime &.{.{