Doc string for the examples of datasets

[zhangguanheng66]

Add the example doc strings to torchtext.datasets.

[codecov[bot]]

Codecov Report

Merging #1241 (918442b) into master (735d5e7) will decrease coverage by 1.83%. The diff coverage is 93.54%.

@@            Coverage Diff             @@
##           master    #1241      +/-   ##
==========================================
- Coverage   80.76%   78.93%   -1.84%     
==========================================
  Files          67       68       +1     
  Lines        3624     3655      +31     
==========================================
- Hits         2927     2885      -42     
- Misses        697      770      +73     

Impacted Files	Coverage Δ
torchtext/data/datasets_utils.py	90.99% <93.33%> (+0.36%)	:arrow_up:
torchtext/data/datasets_example_doc.py	100.00% <100.00%> (ø)
torchtext/vocab.py	67.93% <0.00%> (-22.07%)	:arrow_down:
torchtext/utils.py	72.45% <0.00%> (-4.20%)	:arrow_down:

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[joecummings]

We link to the main page for these datasets, so there's no need for us to also include an example.