Fix and document distance transform watershed for 2D pmaps/images

[qin-yu]

From my understanding, stacked is just for per-slice processing of 3D pmaps, but for 2D and 3D whole-volume watershed we don't use anything stacked-related.

I documented this in the docstring and also improved the overall documentation as I receive many questions regarding this.

[codecov]

Codecov Report

Attention: Patch coverage is 62.93103% with 43 lines in your changes missing coverage. Please review.

Project coverage is 66.41%. Comparing base (8aa7747) to head (b9e5a70).
Report is 10 commits behind head on plantsegv2.

Files with missing lines	Patch %	Lines
plantseg/viewer_napari/widgets/prediction.py	29.16%	34 Missing ⚠️
plantseg/viewer_napari/widgets/segmentation.py	36.36%	7 Missing ⚠️
plantseg/viewer_napari/widgets/dataprocessing.py	0.00%	2 Missing ⚠️

Additional details and impacted files

@@              Coverage Diff               @@
##           plantsegv2     #318      +/-   ##
==============================================
+ Coverage       66.35%   66.41%   +0.06%     
==============================================
  Files              66       67       +1     
  Lines            4541     4571      +30     
==============================================
+ Hits             3013     3036      +23     
- Misses           1528     1535       +7     

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