This is not necessary if `filetype plugin indent on` was not used before
`plug#end()`, since then the `FileType` autocmds from there will come
after vim-plug's.
This will issue a warning, and makes handling of this conditional.
This could use `filetype plugin/indent off` to work around this (similar
to the `filetype off` being used), but `runtime/indoff.vim` and
`runtime/ftplugof.vim` will only empty the augroups, and not remove
them.
Fixing the user's config is the best solution anyway, so I think a
warning is good.
