update message when database is locked by another instance (#17481)

* update message when database is locked by another instance

Tell user that a recently-closed instance of darktable may still be
performing background processing prior to final shutdown.

src/common/database.c

@@ -3713,10 +3713,14 @@ void dt_database_show_error(const dt_database_t *db)
           "      click cancel and either use that instance or close it before attempting to rerun darktable \n"
           "      (process ID <i><b>%d</b></i> created the database locks)\n"
           "\n"
-          "  2 - If you can't find a running instance of darktable, try restarting your session or your computer. \n"
+          "  2 - If you closed darktable within the past few minutes, it may still be running in the background \n"
+          "      to export images, update sidecar files, or perform database maintenance. Try again once \n"
+          "      this processing finishes.\n"
+          "\n"
+          "  3 - If you can't find a running instance of darktable, try restarting your session or your computer. \n"
           "      This will close all running programs and hopefully close the databases correctly. \n"
           "\n"
-          "  3 - If you have done this or are certain that no other instances of darktable are running, \n"
+          "  4 - If you have done this or are certain that no other instances of darktable are running, \n"
           "      this probably means that the last instance was ended abnormally. \n"
           "      Click on the \"delete database lock files\" button to remove the files <i>data.db.lock</i> and <i>library.db.lock</i>.  \n"
           "\n\n"