Add one-line docstring to skimage.segmentation (#6843)

* added a docstring to  submodule

* Use more descriptive one-liner

Try to summarize the aim of image segmentation. This might be a bit more
helpful to readers which can already see the "segmentation" in the
module name.

---------

Co-authored-by: Lars Grüter <lagru+github@mailbox.org>
Co-authored-by: Lars Grüter <lagru@mailbox.org>

skimage/segmentation/__init__.py

@@ -1,3 +1,6 @@
+"""Algorithms to partition images into meaningful regions or boundaries.
+"""
+
 from ._expand_labels import expand_labels
 from .random_walker_segmentation import random_walker
 from .active_contour_model import active_contour