Fix errors in relabel_from_one docs; PEP8

Hi guys,

So, um, funny story! My good friend @DocSavage recently pointed out that the documentation in my `relabel_from_one` function (in `skimage/segmentation/_join.py`) is *dramatically* wrong about the length of the forward map array returned. (Though the example is correct.) This PR fixes the docstring. It additionally fixes a couple of PEP8 gaffes (spacing between functions, newline before EOF).

In the longer term, @DocSavage and I discussed that a relabeling based on iterating through the array will be faster in some situations. (e.g. if I want to relabel `array([0, 1, 2e9])`). However, returning the forward map would be problematic. Possible solutions:

 - add a new function, e.g. `relabel_from_one_lowmem`, for these situations. Let users choose the one they want.
 - keep the one function, and add some logic inside it to determine if the input is pathological, like the array above. If so, use the iterating approach. (And return a dummy forward map?) Otherwise, use the current approach.
 - as above, but *never* return the forward map.

Finally, it would be trivial to allow arbitrary start indices and default the offset to 1, but that would require renaming the function to `relabel_from_offset`. Perhaps deprecation would be a good idea, *or* keep both, *or* ignore that function.

Looking forward to hearing your thoughts!