FIX: Address non-int DeprecationWarning from NumPy

This PR fixes the following warning being raised by NumPy from `relabel_sequential`, due to `m` not being the proper type if `label_field` was input as floating point. This was raised by the final test case in the suite.
```
skimage/segmentation/_join.py:125: DeprecationWarning: using a non-integer number instead of an integer will result in an error in the future
  forward_map = np.zeros(m+1, int)
```
I also fixed a discrepancy in the documentation.

Submitting this separately from the non-integer indexing fixes, as this is a different DeprecationWarning.