Documentation improvements for Blob.diff and Blob.diff_to_buffer

I noticed the documentation not rendering properly for these methods. The first commit should fix this and the second attempts to improve the formatting using the parameter lists that Sphinx supports, which have already been used for `pygit2.clone_repository`.

I have specified the return type with `rtype` but perhaps this is redundant as it is shown at the end of the signature.

I also don't really know what to list as the type for the `GIT_DIFF_*` flags.

I hope it helps,
Michael