[Docs] Update usage doc regarding generate fn

[patrickvonplaten]

Update model.generate() docs

[codecov-io]

Codecov Report

Merging #3504 into master will decrease coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #3504      +/-   ##
==========================================
- Coverage   77.79%   77.76%   -0.03%     
==========================================
  Files         100      100              
  Lines       17051    17051              
==========================================
- Hits        13265    13260       -5     
- Misses       3786     3791       +5     

Impacted Files	Coverage Δ
src/transformers/tokenization_utils.py	90.65% <0.00%> (-0.84%)	⬇️
src/transformers/modeling_tf_utils.py	88.32% <0.00%> (+0.17%)	⬆️

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[LysandreJik]

Cool, this is nice! Thanks!