Custom variable descriptions

[frances-h]

Allow custom descriptions to be set directly on a variable and used in feature descriptions.

• Descriptions default to 'the "[variable name]"' and can be set with variable.description
• Descriptions must be set prior to feature generation in order for them to propagate

[codecov]

Codecov Report

Merging #1207 into main will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #1207   +/-   ##
=======================================
  Coverage   98.63%   98.63%           
=======================================
  Files         134      134           
  Lines       14284    14309   +25     
=======================================
+ Hits        14089    14114   +25     
  Misses        195      195           

Impacted Files	Coverage Δ
featuretools/entityset/deserialize.py	100.00% <100.00%> (ø)
featuretools/entityset/serialize.py	100.00% <100.00%> (ø)
featuretools/feature_base/feature_descriptions.py	98.75% <100.00%> (ø)
...etools/tests/entityset_tests/test_serialization.py	99.62% <100.00%> (+<0.01%)	⬆️
...tests/primitive_tests/test_feature_descriptions.py	100.00% <100.00%> (ø)
...ests/primitive_tests/test_feature_serialization.py	99.34% <100.00%> (ø)
featuretools/variable_types/variable.py	98.80% <100.00%> (+0.05%)	⬆️

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[rwedge]

do we directly test serializing a variable with a custom description?

[rwedge]

let's document the description argument in the Variable docstring

[rwedge]

Looks good!