Allow git_diff_find_similar() to detect renames and copies of small files

See #2735 for the background discussion.
We're not dropping "documentation" here, just removing invalid internal documentation which on top of that is documenting something that is straightforward and therefore not even documented in the rest of the header :)

I can revert that change, but I'm not going to write expensive docs for these functions as part of this PR as I agree this is not the place.