DOC: Complete API for gain_scale

[pllim]

Towards JP-4107

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

On main:

• https://jwst-pipeline.readthedocs.io/en/latest/jwst/gain_scale/index.html

With this patch:

• https://jwst-pipeline--10424.org.readthedocs.build/en/10424/jwst/gain_scale/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 85.76%. Comparing base (95af53b) to head (9d84c76).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #10424   +/-   ##
=======================================
  Coverage   85.76%   85.76%           
=======================================
  Files         372      372           
  Lines       40032    40032           
=======================================
  Hits        34334    34334           
  Misses       5698     5698           

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