Sync README with current public API

Reviewed-on: #1

README.md

@@ -53,21 +53,23 @@ Execute the `tray_example` application:
 
 ## API
 
-Tray structure defines an icon and a menu.
-Menu is a NULL-terminated array of items.
-Menu item defines menu text, menu checked and disabled (grayed) flags and a
-callback with some optional context pointer.
+A tray is defined by an icon, an optional tooltip and a menu.
+The menu is a NULL-terminated array of items.
+Each menu item can be disabled, checked, represented as a checkbox and invoke a
+callback with an optional context pointer.
 
 ```c
 struct tray {
-  char *icon;
+  const char *icon;
+  char *tooltip;
   struct tray_menu *menu;
 };
 
 struct tray_menu {
-  char *text;
+  const char *text;
   int disabled;
   int checked;
+  int checkbox;
 
   void (*cb)(struct tray_menu *);
   void *context;
@@ -76,15 +78,23 @@ struct tray_menu {
 };
 ```
 
-* `int tray_init(struct tray *)` - creates tray icon. Returns -1 if tray icon/menu can't be created.
-* `void tray_update(struct tray *)` - updates tray icon and menu.
-* `int tray_loop(int blocking)` - runs one iteration of the UI loop. Returns -1 if `tray_exit()` has been called.
-* `void tray_exit()` - terminates UI loop.
+* `int tray_init(struct tray *)` - creates the tray icon and its menu. Returns `-1` if the tray icon or menu cannot be created.
+* `void tray_update(struct tray *)` - updates the tray icon, tooltip and menu state.
+* `int tray_loop(int blocking)` - runs one iteration of the UI loop. Returns `-1` after `tray_exit()` has been called.
+* `void tray_exit(void)` - terminates the UI loop and cleans up tray resources.
 
 All functions are meant to be called from the UI thread only.
 
-Menu arrays must be terminated with a NULL item, e.g. the last item in the
-array must have text field set to NULL.
+Menu arrays must be terminated with a NULL item, i.e. the last item in the
+array must have `text == NULL`.
+
+### Notes
+
+* `tooltip` is optional.
+* A separator is created by using `text = "-"`.
+* Some behavior may depend on the underlying backend or desktop environment.
+  Keep the public API generic and treat platform-specific interaction details as
+  backend-specific implementation behavior.
 
 ## License