"directory name.Yet a" needs spaces after '.'.

The text is decent English and clear enough sentence by sentence, but the reality and hence the text as a whole is confusing.

I wonder if it would be possible to make a little table

                    dest
                    ----   
Src     file         empty-dir    non-empty-dir
----    -----------------------------------------
file |            |            |

dir

with the behavior for each combination indicated, 'success' or 'OSError', with two lines prefixed with Unix, Win where they differ. Then the differences would be much more obvious.

(A separate issue: Patch says that Windows currently raises a different error in one situation. I think it would be better -- in a future version -- if that were caught and reraised as OSError also.)