>>951
こいつを ~/.vim/ftdetect/help.vim に置いてみろ

function! s:IsInRTP(afile)
return index(map(split(&rtp, ','), 'resolve(v:val)'), resolve(a:afile)) != -1
endfunction

augroup ftdetectvimhelp
au!
au BufRead,BufNewFile */doc/* if s:IsInRTP(expand('<afile>:p:h:h')) |
\ setl ft=help |
\ endif
augroup END