Add callback typedefs to the API documentation.

[cirosantilli]

Callback typedefs are not being put in the documentation. E.g.: https://libgit2.github.com/libgit2/#HEAD/group/diff/git_diff_blob_to_buffer uses git_diff_file_cb, which it defined and documented publicly at: https://github.com/libgit2/libgit2/blob/master/include/git2/diff.h#L416. It would be really helpful to have links to those callbacks to understand how to call the functions that use them.

[carlosmn]

Most of this has been taken care of with a0143c32 which links those callback types with documentation and exposes them through the search.

[cirosantilli]

Carlos, feel free to close this if you consider this solved: I'm unable to check it right now.