docs: add warning about async simulation

[CWHer]

• I have marked all applicable categories:
  • exception-raising fix
  • algorithm implementation fix
  • documentation modification
  • new feature
• I have reformatted the code using make format (required)
• I have checked the code using make commit-checks (required)
• If applicable, I have mentioned the relevant/related issue(s)
• If applicable, I have listed every items in this Pull Request below

[codecov-commenter]

Codecov Report

Merging #705 (86ce32f) into master (99c99bb) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #705   +/-   ##
=======================================
  Coverage   93.42%   93.42%           
=======================================
  Files          72       72           
  Lines        4894     4895    +1     
=======================================
+ Hits         4572     4573    +1     
  Misses        322      322           

Flag	Coverage Δ
unittests	93.42% <100.00%> (+<0.01%)	⬆️

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Impacted Files	Coverage Δ
tianshou/trainer/base.py	96.90% <100.00%> (+0.01%)	⬆️

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