[T5] Add training documenation

[patrickvonplaten]

• Fixes T5 docstring regarding pretraining
• Add detailed description on how to process input and target for T5 training

[codecov-io]

Codecov Report

Merging #3507 into master will increase coverage by 0.00%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #3507   +/-   ##
=======================================
  Coverage   77.79%   77.80%           
=======================================
  Files         100      100           
  Lines       17051    17051           
=======================================
+ Hits        13265    13266    +1     
+ Misses       3786     3785    -1     

Impacted Files	Coverage Δ
src/transformers/modeling_bart.py	97.58% <ø> (ø)
src/transformers/modeling_t5.py	81.29% <ø> (ø)
src/transformers/modeling_tf_t5.py	94.98% <ø> (ø)
src/transformers/modeling_utils.py	91.94% <0.00%> (+0.13%)	⬆️

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