Clarification of collectstatic.py warning message.

I am not a programmer, so I may not have completed this small task perfectly (I had trouble figuring out how to break the text line so it did not extend beyond 80 chars), but it seems that the collectstatic warning message should be more clear: it will not *always* overwrite files; it will only replace existing default files with new default files. So, it will only overwrite default files that the user has modified. This is hard to describe in one sentence, but I have attempted to do so.