[Enhance] Add doc for S3DIS semantic segmentation data

[Wuziyi616]

Follow #743

[codecov]

Codecov Report

Merging #747 (13d9fde) into master (2f95eb7) will increase coverage by 0.04%.
The diff coverage is 33.33%.

@@            Coverage Diff             @@
##           master     #747      +/-   ##
==========================================
+ Coverage   48.91%   48.96%   +0.04%     
==========================================
  Files         208      208              
  Lines       15866    15880      +14     
  Branches     2546     2538       -8     
==========================================
+ Hits         7761     7775      +14     
+ Misses       7609     7603       -6     
- Partials      496      502       +6     

Flag	Coverage Δ
unittests	48.96% <33.33%> (+0.04%)	⬆️

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Impacted Files	Coverage Δ
mmdet3d/models/detectors/single_stage.py	58.62% <12.50%> (-17.57%)	⬇️
...et3d/models/dense_heads/anchor_free_mono3d_head.py	65.31% <33.33%> (+6.67%)	⬆️
mmdet3d/models/dense_heads/fcos_mono3d_head.py	12.10% <66.66%> (+0.13%)	⬆️
mmdet3d/models/detectors/voxelnet.py	33.87% <100.00%> (ø)
mmdet3d/datasets/scannet_dataset.py	78.51% <0.00%> (-9.34%)	⬇️
mmdet3d/models/detectors/imvotenet.py	26.66% <0.00%> (ø)
mmdet3d/models/dense_heads/groupfree3d_head.py	22.44% <0.00%> (ø)

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