Missing doc strings

[gsheni]

• Added some doc strings in binary_transform.py
• Also cleaned up some other doc strings.

[codecov-io]

Codecov Report

Merging #23 into master will increase coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master      #23      +/-   ##
==========================================
+ Coverage   86.85%   86.86%   +0.01%     
==========================================
  Files          73       73              
  Lines        6777     6778       +1     
==========================================
+ Hits         5886     5888       +2     
+ Misses        891      890       -1

Impacted Files	Coverage Δ
featuretools/primitives/aggregation_primitives.py	92.1% <ø> (ø)	⬆️
featuretools/primitives/binary_transform.py	83.83% <ø> (ø)	⬆️
featuretools/primitives/transform_primitive.py	96.78% <ø> (-0.05%)	⬇️
featuretools/entityset/serialization.py	17.64% <0%> (-1.2%)	⬇️
.../feature_function_tests/test_transform_features.py	85.15% <0%> (-0.16%)	⬇️
featuretools/tests/testing_utils/mock_ds.py	94.54% <0%> (ø)	⬆️
...turetools/tests/feature_function_tests/__init__.py	100% <0%> (ø)	⬆️
...computational_backends/calculate_feature_matrix.py	96.61% <0%> (ø)	⬆️
... and 63 more

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