fix(api): mark nvim__complete_set as experimental #28579

Problem:
nvim_complete_set was added in 5ed55ff14c
but needs more bake time.

Solution:
Rename it, mark it as experimental.

src/nvim/popupmenu.c

@@ -732,7 +732,7 @@ static void pum_adjust_info_position(win_T *wp, int height, int width)
   win_config_float(wp, wp->w_config);
 }
 
-/// Used for nvim_complete_set
+/// Used for nvim__complete_set
 ///
 /// @param selected the selected compl item.
 /// @parma info     Info string.