Since we now always build the library with cdecl calling conventions,
our callbacks should be decorated as such so that users will not be able
to provide callbacks defined with other calling conventions.

The `GIT_CALLBACK` macro will inject the `__cdecl` attribute as
appropriate.

Otherwise docurium/clang chokes on the types, and ignores the documentation comments altogether.

The options parameter in both git_stash_apply and git_stash_pop can
be NULL. They should be flagged as such in the documentation.