vim-patch:9.1.0171: Small split-move related improvements

Problem:  small improvements can be made to split-move related
          functions.
Solution: apply them (Sean Dewar):

Some of these changes were already applied to Nvim.
Here are the ones which were missing:

- Improve some doc comments (frame_flatten should still work for non-current
  tabpages, despite the topframe check, which looks benign, though I'm unsure if
  it's still needed; see vim/vim#2467).

- f_win_splitmove should check_split_disallowed on wp, not targetwin, as that's
  what win_splitmove checks (though it's probably unnecessary to check
  b_locked_split at all; see vim/vim#14109, which I hope to get around to
  finishing at some point).

- Apply the winframe_restore comment changes, and remove win_comp_pos from after
  winframe_restore in win_splitmove, as it shouldn't be necessary (no need to
  remove it from nvim_win_set_config too, as it was already omitted).
  Move win_append after winframe_restore in win_splitmove to match Vim.

closes: vim/vim#14185

5cac1a9bee

src/nvim/eval/window.c

@@ -701,8 +701,8 @@ void f_win_splitmove(typval_T *argvars, typval_T *rettv, EvalFuncData fptr)
     size = (int)tv_dict_get_number(d, "size");
   }
 
-  // Check if we can split the target before we bother switching windows.
-  if (is_aucmd_win(wp) || text_or_buf_locked() || check_split_disallowed(targetwin) == FAIL) {
+  // Check if we're allowed to continue before we bother switching windows.
+  if (is_aucmd_win(wp) || text_or_buf_locked() || check_split_disallowed(wp) == FAIL) {
     return;
   }