Fix documentation and naming of torchtext.experimental.vocab factory functions.

[cpuhrsch]

a) The documentation doesn't clearly state that one factory function is meant to be used to construct a Vocabulary from a dataset (e.g. AG_NEWS) and another is meant to be used to load a vocabulary from a text file that contains a list of tokens (as you might construct when filtering and sorting text files on the command line). b) factory function vocab_from_file_object doesn't exist.

[zhangguanheng66]

there was a back and forth that the names of the functions were changed from load_from_file_object to load_from_file

[codecov[bot]]

Codecov Report

Merging #991 into master will not change coverage. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #991   +/-   ##
=======================================
  Coverage   77.70%   77.70%           
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  Files          44       44           
  Lines        3099     3099           
=======================================
  Hits         2408     2408           
  Misses        691      691           

Impacted Files	Coverage Δ
torchtext/experimental/transforms.py	87.50% <ø> (ø)
torchtext/experimental/vocab.py	100.00% <100.00%> (ø)

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[rshraga]

closing stale PR