Powerline: Mention alternative to notexists symbol

I noticed, that in some environments the default
g:airline_symbols[notexists] just does not show up, even when the same
font in the gui works. So let's at least mention a possible alternative,
that you can easily use in your vimrc:

```vim
  if !exists("g:airline_symbols")
    let g:airline_symbols = {}
  endif
  let g:airline_symbols.notexists = "\u2204"
```

doc/airline.txt

@@ -386,6 +386,7 @@ its contents. >
   let g:airline_symbols.paste = '∥'
   let g:airline_symbols.spell = 'Ꞩ'
   let g:airline_symbols.notexists = 'Ɇ'
+  let g:airline_symbols.notexists = '∄'
   let g:airline_symbols.whitespace = 'Ξ'
 
   " powerline symbols