Add unit tests to enforce primitive docstring verb usage

[sbadithe]

Fixes #2199

-- Adds unit test that checks if primitive's docstring begins with verb in set of valid_verbs.
-- Rewords docstrings for primitives that would fail test above.

[codecov]

Codecov Report

Merging #2200 (ac259ec) into main (d2f0092) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #2200   +/-   ##
=======================================
  Coverage   99.29%   99.29%           
=======================================
  Files         145      146    +1     
  Lines       17556    17567   +11     
=======================================
+ Hits        17432    17443   +11     
  Misses        124      124           

Impacted Files	Coverage Δ
...aturetools/primitives/standard/binary_transform.py	100.00% <ø> (ø)
...imitives/standard/datetime_transform_primitives.py	100.00% <ø> (ø)
...retools/primitives/standard/transform_primitive.py	100.00% <ø> (ø)
...s/primitive_tests/test_all_primitive_docstrings.py	100.00% <100.00%> (ø)

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[gsheni]

lgtm