Document better sign options everywhere

In both `:help ale-*` somewhere and in the FAQ we should come up with some better defaults for signs that will work more often than not. We cannot change the sign defaults without breaking ALE on certain machines, but we can suggest that people try something like this.

```vim
scriptencoding utf-8

let g:ale_sign_error = '◉'
let g:ale_sign_warning = '◉'
highlight ALEErrorSign ctermfg=9 ctermbg=15 guifg=#C30500 guibg=#F5F5F5
highlight ALEWarningSign ctermfg=11 ctermbg=15 guifg=#ED6237 guibg=#F5F5F5
```