Add note on line break behavior for empty rope

[msmoiz]

I tripped over the fact that an empty buffer will return a line from the lines iterator. It was a bit unexpected since it deviates from the behavior of str::lines (which returns no lines in that case). That is probably fine (and difficult to change at this point anyway), but I think a bit more documentation might be useful here, so I added it. The diff looks more involved than it actually is; all I did was add a sentence to the paragraph.

Thanks for your work on this project!