Update documentation about objectives in AutoML

[angela97lin]

Closes #1211 by clarifying in AutoMLSearch docstring and AutoML docs what the objective parameter is used for.

Updated API reference and updated automl guide

[codecov]

Codecov Report

Merging #1222 into main will increase coverage by 8.32%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #1222      +/-   ##
==========================================
+ Coverage   91.60%   99.92%   +8.32%     
==========================================
  Files         200      200              
  Lines       12468    12468              
==========================================
+ Hits        11421    12459    +1038     
+ Misses       1047        9    -1038     

Impacted Files	Coverage Δ
evalml/automl/automl_search.py	99.58% <ø> (+0.41%)	⬆️
...s/prediction_explanations_tests/test_algorithms.py	100.00% <0.00%> (+1.11%)	⬆️
evalml/tests/component_tests/test_components.py	100.00% <0.00%> (+1.25%)	⬆️
evalml/tests/component_tests/test_utils.py	100.00% <0.00%> (+3.57%)	⬆️
evalml/tests/pipeline_tests/test_pipelines.py	100.00% <0.00%> (+3.71%)	⬆️
...derstanding/prediction_explanations/_algorithms.py	97.14% <0.00%> (+4.28%)	⬆️
evalml/utils/gen_utils.py	99.09% <0.00%> (+5.40%)	⬆️
evalml/pipelines/pipeline_base.py	100.00% <0.00%> (+6.14%)	⬆️
evalml/tests/utils_tests/test_dependencies.py	100.00% <0.00%> (+6.25%)	⬆️
... and 17 more

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[bchen1116]

LGTM! Docs look clean.

[freddyaboulton]

@angela97lin Looks good to me!