Add missing arguments and docs

[ethanwharris]

What does this PR do?

Adds missing multi-label arguments following #230

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[codecov]

Codecov Report

Merging #245 (63839f5) into master (7486c33) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #245      +/-   ##
==========================================
- Coverage   87.52%   87.51%   -0.01%     
==========================================
  Files          59       59              
  Lines        3053     3051       -2     
==========================================
- Hits         2672     2670       -2     
  Misses        381      381              

Flag	Coverage Δ
unittests	87.51% <100.00%> (-0.01%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
flash/text/classification/data.py	82.47% <ø> (ø)
flash/text/classification/model.py	93.75% <ø> (-0.19%)	⬇️
flash/core/classification.py	92.85% <100.00%> (+0.79%)	⬆️
flash/tabular/classification/model.py	87.50% <100.00%> (+0.54%)	⬆️
flash/vision/classification/model.py	96.00% <100.00%> (-1.06%)	⬇️

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[tchaton]

LGTM ! nice.

[edgarriba]

LGTM