Fix docstrings for Blob.diff & diff_to_buffer

They were both missing a closing parenthesis which stop Sphinx from
interpreting and formatting them properly.
Improve diff & diff_to_buffer doc formatting

We switch to a parameter list for both functions and add a return type.
We also remove the indentation from the second line of the explanation
which was causing Sphinx to return it as two lines instead of one
continuous one.

We also added return types.

I am not sure of the type of the GIT_DIFF* flags so I have not included
that.
Merge remote-tracking branch 'michaeljones/documentation'