docs: Add docstrings to _ModelConfig methods

[matthewfeickert]

Description

To satisfy pydocstyle in PR #1403 docstrings need to be added to pyhf.pdf._ModelConfig, so these are added here.

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• Tests are passing
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* Add docstrings and examples to pyhf.pdf._ModelConfig and its methods

[matthewfeickert]

Relevant RTD build section: https://pyhf--1420.org.readthedocs.build/en/1420/_generated/pyhf.pdf._ModelConfig.html

[codecov]

Codecov Report

Merging #1420 (3c1145c) into master (2efe0d2) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1420   +/-   ##
=======================================
  Coverage   97.58%   97.58%           
=======================================
  Files          63       63           
  Lines        3886     3886           
  Branches      555      555           
=======================================
  Hits         3792     3792           
  Misses         55       55           
  Partials       39       39           

Flag	Coverage Δ
contrib	25.03% <ø> (ø)
unittests	97.34% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/pyhf/pdf.py	96.40% <ø> (ø)

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