Update doc, now that FIL supports multi-class classification

[hcho3]

Closes #3627

[codecov-io]

Codecov Report

Merging #3634 (f52cbeb) into branch-0.19 (14bd6c1) will increase coverage by 0.10%.
The diff coverage is n/a.

@@               Coverage Diff               @@
##           branch-0.19    #3634      +/-   ##
===============================================
+ Coverage        80.72%   80.82%   +0.10%     
===============================================
  Files              228      228              
  Lines            17629    17751     +122     
===============================================
+ Hits             14231    14348     +117     
- Misses            3398     3403       +5     

Flag	Coverage Δ
dask	45.27% <ø> (+0.31%)	⬆️
non-dask	73.10% <ø> (+0.15%)	⬆️

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

Impacted Files	Coverage Δ
...ython/cuml/dask/ensemble/randomforestclassifier.py	56.25% <ø> (+1.12%)	⬆️
python/cuml/ensemble/randomforestclassifier.pyx	73.40% <ø> (ø)
python/cuml/fil/fil.pyx	91.83% <ø> (ø)
python/cuml/linear_model/linear_regression.pyx	88.23% <0.00%> (-3.53%)	⬇️
python/cuml/dask/solvers/cd.py	100.00% <0.00%> (ø)
python/cuml/common/numba_utils.py	0.00% <0.00%> (ø)
python/cuml/internals/global_settings.py	100.00% <0.00%> (ø)
python/cuml/dask/preprocessing/encoders.py	100.00% <0.00%> (ø)
python/cuml/metrics/_ranking.py	98.57% <0.00%> (+0.02%)	⬆️
python/cuml/preprocessing/encoders.py	95.10% <0.00%> (+0.02%)	⬆️
... and 37 more

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[dantegd]

lgtm

[dantegd]

@gpucibot merge