doc improvements

[stas00]

a few documentation improvements - one variable name consistency rename, and the rest are small tweaks with one clarification.

[codecov]

Codecov Report

Merging #5688 into master will decrease coverage by 0.24%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #5688      +/-   ##
==========================================
- Coverage   77.01%   76.76%   -0.25%     
==========================================
  Files         128      146      +18     
  Lines       21615    25983    +4368     
==========================================
+ Hits        16646    19945    +3299     
- Misses       4969     6038    +1069     

Impacted Files	Coverage Δ
src/transformers/__init__.py	99.24% <ø> (+0.11%)	⬆️
src/transformers/benchmark/benchmark.py	74.01% <ø> (+5.16%)	⬆️
src/transformers/benchmark/benchmark_args.py	86.04% <ø> (+0.68%)	⬆️
src/transformers/benchmark/benchmark_args_tf.py	87.50% <ø> (ø)
src/transformers/benchmark/benchmark_args_utils.py	89.13% <ø> (-7.75%)	⬇️
src/transformers/benchmark/benchmark_tf.py	61.53% <ø> (ø)
src/transformers/benchmark/benchmark_utils.py	69.50% <ø> (-3.60%)	⬇️
src/transformers/configuration_albert.py	100.00% <ø> (ø)
src/transformers/configuration_auto.py	93.18% <ø> (+0.32%)	⬆️
src/transformers/configuration_bart.py	93.75% <ø> (+0.41%)	⬆️
... and 109 more

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[JetRunner]

Thanks, LGTM