pacdiff: Improve -s/--sudo documentation slightly

When the -s/--sudo option was added recently, it was a helpful addition. I guessed (correctly) that it was going to use sudo for operations like mv, rm, etc. But it wasn't immediately obvious until I used it (and dug into the source code) that it would also use sudoedit for file editing rather than simply sudo $diffprog. This was initially jarring because one isn't operating on the files themselves anymore (original or pacnew), but temp files. I don't have a problem with this, I just think it could be more clearly documented in usage() and on the man page.

As I don't have permission to fork or create a Merge Request here, please find in the expandable block below a proposed patch for this change to pacdiff.