No documentation for sort modes

[ghost]

I haven't seen any clear documentation on the sort mode options, i.e. Rugged::SORT_{NONE,TOPO,...}. Could this please be mentioned somewhere, perhaps in the Walker class docs?

[spraints]

I poked around a bit and found some docs in the libgit2 code (https://github.com/libgit2/libgit2/blob/a39f18ac770e1a4bac72212236614db57c1de5be/include/git2/revwalk.h#L23-L53).