DOC: Complete API for barshadow

[pllim]

Towards JP-4107

This PR addresses barshadow portion of #9793 . Only touch docs so should not need RT.

On main:

• https://jwst-pipeline.readthedocs.io/en/stable/jwst/barshadow/index.html

With this patch:

• https://jwst-pipeline--10258.org.readthedocs.build/en/10258/jwst/barshadow/index.html

Tasks

• If you have a specific reviewer in mind, tag them.
• add a build milestone, i.e. Build 12.0 (use the latest build if not sure)
• Does this PR change user-facing code / API? (if not, label with no-changelog-entry-needed)
  • write news fragment(s) in changes/: echo "changed something" > changes/<PR#>.<changetype>.rst (see changelog readme for instructions)
    • if your change breaks step-level or public API (as defined in the docs), also add a changes/<PR#>.breaking.rst news fragment
  • update or add relevant tests
  • update relevant docstrings and / or docs/ page
  • start a regression test and include a link to the running job (click here for instructions)
    • Do truth files need to be updated ("okified")?
      • after the reviewer has approved these changes, run okify_regtests to update the truth files
• if a JIRA ticket exists, make sure it is resolved properly

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.33%. Comparing base (bbf1e5f) to head (f88ed6f).
⚠️ Report is 79 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #10258      +/-   ##
==========================================
- Coverage   85.98%   85.33%   -0.65%     
==========================================
  Files         368      370       +2     
  Lines       38457    39132     +675     
==========================================
+ Hits        33066    33393     +327     
- Misses       5391     5739     +348     

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