Update documentation about the modification of train_cfg/test_cfg

[hhaAndroid]

Update documentation about the modification of train_cfg/test_cfg

resolve #4470

[codecov]

Codecov Report

Merging #4489 (a3dae02) into master (e1dd6e2) will increase coverage by 0.19%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #4489      +/-   ##
==========================================
+ Coverage   61.81%   62.00%   +0.19%     
==========================================
  Files         241      241              
  Lines       18357    18357              
  Branches     3075     3075              
==========================================
+ Hits        11347    11383      +36     
+ Misses       6454     6416      -38     
- Partials      556      558       +2     

Flag	Coverage Δ
unittests	61.96% <ø> (+0.17%)	⬆️

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Impacted Files	Coverage Δ
mmdet/models/roi_heads/mask_heads/fcn_mask_head.py	71.77% <0.00%> (+1.22%)	⬆️
mmdet/models/roi_heads/mask_heads/maskiou_head.py	100.00% <0.00%> (+5.37%)	⬆️
mmdet/models/roi_heads/test_mixins.py	53.43% <0.00%> (+8.39%)	⬆️
mmdet/models/roi_heads/mask_scoring_roi_head.py	87.50% <0.00%> (+32.14%)	⬆️

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