docs: Clarify absolute/relative for histosys

[kratsg]

Pull Request Description

Resolves #1718.

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• Tests are passing
• "WIP" removed from the title of the pull request
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* Clarify the schema data for histosys is absolute and not relative counts in the
  Correlated Shape (histosys) documentation example for likelihood specification.

[codecov]

Codecov Report

Base: 98.24% // Head: 98.24% // No change to project coverage 👍

Coverage data is based on head (8c7f0c6) compared to base (e366eb9).
Patch has no changes to coverable lines.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1971   +/-   ##
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  Coverage   98.24%   98.24%           
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  Files          68       68           
  Lines        4378     4378           
  Branches      726      726           
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  Hits         4301     4301           
  Misses         45       45           
  Partials       32       32           

Flag	Coverage Δ
contrib	26.58% <ø> (ø)
doctest	60.57% <ø> (ø)
unittests-3.10	96.13% <ø> (ø)
unittests-3.7	96.12% <ø> (ø)
unittests-3.8	96.16% <ø> (ø)
unittests-3.9	96.18% <ø> (ø)

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[matthewfeickert]

@kratsg thanks for this and I think the clarification is good. I have a minor suggestion though.

[matthewfeickert]

Relevant RTD build: https://pyhf--1971.org.readthedocs.build/en/1971/likelihood.html#correlated-shape-histosys