'titleold': simplify behavior

- default 'titleold' to empty
- set title on exit if 'title' is enabled and 'titleold' is non-empty
- update docs

src/nvim/options.lua

@@ -2498,7 +2498,7 @@ return {
       no_mkrc=true,
       vi_def=true,
       varname='p_titleold',
-      defaults={if_true={vi=N_("Thanks for flying Vim")}}
+      defaults={if_true={vi=N_("")}}
     },
     {
       full_name='titlestring',