Fixup ert plugin documentation

[larsevj]

• PR title captures the intent of the changes, and is fitting for release notes.
• Added appropriate release note label
• Commit history is consistent and clean, in line with the contribution guidelines.
• Make sure unit tests pass locally after every commit (git rebase -i main --exec 'pytest tests/ert/unit_tests -n logical -m "not integration_test"')

When applicable

• When there are user facing changes: Updated documentation
• New behavior or changes to existing untested code: Ensured that unit tests are added (See Ground Rules).
• Large PR: Prepare changes in small commits for more convenient review
• Bug fix: Add regression test for the bug
• Bug fix: Create Backport PR to latest release

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.01%. Comparing base (1315c55) to head (f003952).
Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #9013      +/-   ##
==========================================
+ Coverage   90.86%   91.01%   +0.15%     
==========================================
  Files         349      349              
  Lines       21621    21645      +24     
==========================================
+ Hits        19645    19701      +56     
+ Misses       1976     1944      -32     

Flag	Coverage Δ
cli-tests	39.20% <ø> (+0.07%)	⬆️
gui-tests	72.91% <ø> (+0.12%)	⬆️
performance-tests	49.69% <ø> (+0.02%)	⬆️
unit-tests	79.70% <ø> (+0.05%)	⬆️

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