Add AlbertForPretraining to doc

[sgugger]

Document models that were absent.

[codecov]

Codecov Report

Merging #5914 into master will increase coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #5914      +/-   ##
==========================================
+ Coverage   78.49%   78.51%   +0.02%     
==========================================
  Files         146      146              
  Lines       26214    26214              
==========================================
+ Hits        20577    20583       +6     
+ Misses       5637     5631       -6     

Impacted Files	Coverage Δ
src/transformers/generation_utils.py	97.11% <0.00%> (+0.28%)	⬆️
src/transformers/file_utils.py	81.49% <0.00%> (+0.29%)	⬆️
src/transformers/generation_tf_utils.py	86.21% <0.00%> (+1.00%)	⬆️

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