improve documentation for c-related fields

The documentation for `c-sources` should state that the sources need to be listed [in dependency order](https://ghc.haskell.org/trac/ghc/ticket/13786#comment:3).

Also, it would be nice to have a bit more clarity on the various include-file-related options. Given that `include-files` is [ignored by GHC](https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/ffi-chap.html#using-header-files), that should probably be noted in the Cabal documentation, too. And I'm unclear when `install-includes` would be needed, if ever. My solution has been to just put include files in `extra-source-files`, but it would be nice if the documentation offered more guidance on this.

Thanks!

Start with the Cabal documentation for c-sources, include-files, and install-includes, then compare the linked GHC FFI documentation. Document dependency ordering for c-sources, note that GHC ignores include-files, and clarify when install-includes is needed versus extra-source-files.

Written by the indexing model from the issue text.