Document evalml/preprocessing/ code

[angela97lin]

Closes #393

[codecov]

Codecov Report

Merging #687 into master will increase coverage by 11.30%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           master     #687       +/-   ##
===========================================
+ Coverage   87.78%   99.09%   +11.30%     
===========================================
  Files         139      139               
  Lines        4954     4954               
===========================================
+ Hits         4349     4909      +560     
+ Misses        605       45      -560     

Impacted Files	Coverage Δ
evalml/preprocessing/utils.py	100.00% <ø> (ø)
evalml/tests/component_tests/test_components.py	100.00% <0.00%> (+0.60%)	⬆️
evalml/automl/auto_base.py	96.42% <0.00%> (+0.97%)	⬆️
evalml/tests/pipeline_tests/test_pipelines.py	99.62% <0.00%> (+3.39%)	⬆️
evalml/tests/component_tests/test_utils.py	96.42% <0.00%> (+3.57%)	⬆️
evalml/pipelines/pipeline_base.py	99.03% <0.00%> (+5.79%)	⬆️
evalml/tests/utils_tests/test_dependencies.py	100.00% <0.00%> (+6.25%)	⬆️
...ts/automl_tests/test_auto_classification_search.py	100.00% <0.00%> (+15.66%)	⬆️
.../tests/automl_tests/test_auto_regression_search.py	100.00% <0.00%> (+25.77%)	⬆️
evalml/tests/pipeline_tests/test_graphs.py	100.00% <0.00%> (+26.02%)	⬆️
... and 14 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update c994d60...3bc249f. Read the comment docs.

[dsherry]

@angela97lin reminder to please move to Review when you request a review!

[dsherry]

One error but otherwise lgtm!