Vint warns if there is no augroup in ftdetect scripts

[przepompownia]

Both manual mention that there are no need to wrap commands by an additional autocmd.

[telemachus]

To stay this even more strongly, I think that :help ftdetect implies that scripts should not wrap autocommands in an augroup in a ftdetect file.

There have definitely been problems when people follow the false warning. E.g, fatih/vim-go#1645 mentioned in #264.

Would you be open to pull requests for these two changes?

• Remove warning about missing augroup in ftdetect scripts.
• Add a warning if someone (mistakenly) uses an augroup in a ftdetect script? (As suggested by @thinca in the issue above.)