docs: improve 'insertmode' emulation #18962

The current emulation script enters Insert mode much too frequently.
Using only BufWinEnter seems to be a closer simulation.

Also add a few more mappings.

runtime/doc/vim_diff.txt

@@ -550,10 +550,31 @@ Options:
   *'imstatusfunc'* *'imsf'*
   *'insertmode'* *'im'* Use the following script to emulate 'insertmode':
 >
-      autocmd VimEnter,CmdlineLeave,WinEnter,WinScrolled,BufEnter * silent! if &modifiable | startinsert | endif
+    autocmd BufWinEnter * startinsert
-      inoremap <Esc> <Nop>
+    inoremap <Esc> <C-X><C-Z><C-]>
-      inoremap <C-L> <Esc>
+    inoremap <C-C> <C-X><C-Z>
-      nnoremap <Esc> i
+    inoremap <C-L> <C-X><C-Z><C-]><Esc>
+    inoremap <C-Z> <C-X><C-Z><Cmd>suspend<CR>
+    noremap <C-C> <Esc>
+    snoremap <C-C> <Esc>
+    noremap <C-\><C-G> <C-\><C-N><Cmd>startinsert<CR>
+    cnoremap <C-\><C-G> <C-\><C-N><Cmd>startinsert<CR>
+    inoremap <C-\><C-G> <C-X><C-Z>
+    autocmd CmdWinEnter * noremap <buffer> <C-C> <C-C>
+    autocmd CmdWinEnter * inoremap <buffer> <C-C> <C-C>
+
+    lua << EOF
+      vim.on_key(function(c)
+        if c == '\27' then
+          local mode = vim.api.nvim_get_mode().mode
+          if mode:find('^[nvV\22sS\19]') and vim.fn.getcmdtype() == '' then
+            vim.schedule(function()
+              vim.cmd('startinsert')
+            end)
+          end
+        end
+      end)
+    EOF
 <
   *'macatsui'*
   *'maxcombine'* *'mco'*