DOC: Improve docstrings for Semantic Seg and Feature Extractor

[simongraham]

Modified docstrings so that they synchronised with readthedocs.

Changed:

• Semantic segmentation docs
• Deep feature extractor docs

[codecov]

Codecov Report

Merging #172 (c069c2c) into develop (98a2713) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           develop     #172   +/-   ##
========================================
  Coverage    99.79%   99.79%           
========================================
  Files           40       40           
  Lines         2955     2955           
  Branches       492      492           
========================================
  Hits          2949     2949           
  Misses           1        1           
  Partials         5        5           

Impacted Files	Coverage Δ
tiatoolbox/models/architecture/unet.py	100.00% <ø> (ø)
tiatoolbox/models/architecture/utils.py	100.00% <ø> (ø)
tiatoolbox/models/architecture/vanilla.py	100.00% <ø> (ø)
tiatoolbox/models/controller/semantic_segmentor.py	100.00% <ø> (ø)
tiatoolbox/utils/transforms.py	100.00% <ø> (ø)

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