Feature: improve refresh token usage and docs

[yanyongyu]

related to #125 #126

[codecov]

Codecov Report

Attention: Patch coverage is 6.97674% with 40 lines in your changes missing coverage. Please review.

Project coverage is 17.69%. Comparing base (68ea192) to head (000b6de).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
githubkit/auth/oauth.py	6.97%	40 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #128      +/-   ##
==========================================
- Coverage   17.69%   17.69%   -0.01%     
==========================================
  Files        4834     4834              
  Lines      223288   223331      +43     
==========================================
+ Hits        39512    39515       +3     
- Misses     183776   183816      +40     

Flag	Coverage Δ
unittests	17.69% <6.97%> (-0.01%)	⬇️

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