https://github.com/neovim/neovim/pull/27428 changed the semantics of
callbacks passed to nvim_create_autocmd such that any truthy value will
delete the autocommand (rather than just the literal boolean value
`true`). Update the documentation accordingly and add an entry to
`news.txt`.
The behavior is now consistent between nvim_create_autocmd and
nvim_buf_attach.