Fix daemon persist bug and document instance() (#2185)

Signed-off-by: Haris Gušić <harisgusic.dev@gmail.com>

src/core/flameshotdaemon.cpp

@@ -142,6 +142,18 @@ bool FlameshotDaemon::isThisInstanceHostingWidgets()
     return instance() && !instance()->m_widgets.isEmpty();
 }
 
+/**
+ * @brief Return the daemon instance.
+ *
+ * If this instance of flameshot is the daemon, a singleton instance of
+ * `FlameshotDaemon` is returned. As a side effect`start` will called if it
+ * wasn't called earlier. If this instance of flameshot is not the daemon,
+ * `nullptr` is returned.
+ *
+ * This strategy is used because the daemon needs to receive signals from D-Bus,
+ * for which an instance of a `QObject` is required. The singleton serves as
+ * that object.
+ */
 FlameshotDaemon* FlameshotDaemon::instance()
 {
     // Because we don't use DBus on MacOS, each instance of flameshot is its own
@@ -159,7 +171,7 @@ FlameshotDaemon* FlameshotDaemon::instance()
  */
 void FlameshotDaemon::quitIfIdle()
 {
-    if (m_persist && !instance()) {
+    if (m_persist) {
         return;
     }
     if (!m_hostingClipboard && m_widgets.isEmpty()) {