Update all references to 'label' to 'target' if problem type is not classification

[angela97lin]

Closes #829 by updating API reference to use "target" instead of "label" in non-classification contexts.

I also did some general docstring cleanup (Args --> Arguments, capitalization)

[codecov]

Codecov Report

Merging #1160 into main will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #1160   +/-   ##
=======================================
  Coverage   99.92%   99.92%           
=======================================
  Files         196      196           
  Lines       11995    11995           
=======================================
  Hits        11986    11986           
  Misses          9        9           

Impacted Files	Coverage Δ
evalml/automl/automl_search.py	99.58% <ø> (ø)
evalml/automl/pipeline_search_plots.py	100.00% <ø> (ø)
evalml/data_checks/data_checks.py	100.00% <ø> (ø)
evalml/data_checks/highly_null_data_check.py	100.00% <ø> (ø)
evalml/data_checks/id_columns_data_check.py	100.00% <ø> (ø)
evalml/data_checks/invalid_targets_data_check.py	100.00% <ø> (ø)
evalml/data_checks/label_leakage_data_check.py	100.00% <ø> (ø)
evalml/data_checks/outliers_data_check.py	100.00% <ø> (ø)
evalml/demos/churn.py	100.00% <ø> (ø)
evalml/demos/fraud.py	100.00% <ø> (ø)
... and 31 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 0ae5014...ded3f99. Read the comment docs.

[freddyaboulton]

@angela97lin Thanks for doing this! I left non-blocking comments. The main theme is that sometimes we say target data, target values, or true values in regression contexts. Might be nice to standardize but not blocking as it's pretty minor.

[bchen1116]

LGTM

[dsherry]

@angela97lin thank you for doing this!