Add help for "remove last build" in Manage Jenkins

[AmoghParmar]

Fixes #16688

This pull request adds missing online help documentation for the
“Remove last build” option of the LogRotator build discarder.

The help text explains the behavior, provides an example,
and references Pipeline usage.

Testing done

• Built Jenkins locally from source
• Verified the new help icon appears for the “Remove last build” option in
  Manage Jenkins → Configure System → Build Discarder → Advanced
• Clicked the help icon and confirmed the help text renders correctly

Screenshots (UI changes only)

Before

No online help was available for the “Remove last build” option.

After

A help icon is displayed next to the “Remove last build” option, showing detailed documentation when clicked.

Proposed changelog entries

• Add online help for the “Remove last build” build discarder option.

Proposed changelog category

/label rfe,web-ui

Proposed upgrade guidelines

N/A

Submitter checklist

• The issue, if it exists, is well-described.
• The changelog entries and upgrade guidelines are appropriate.
• There is automated testing or an explanation as to why this change has no tests.
• UI changes do not introduce regressions with Content Security Policy.

Maintainer checklist

• At least two approvals
• No unresolved conversations
• Changelog entries look correct
• Proper labels set

Desired reviewers

@jenkinsci/core-pr-reviewers

[MarkEWaite]

Please restore the pull request template, including the submitter checklist and the maintainer checklist and the changelog description

[MarkEWaite]

Additional comment on the Pipeline example

[AmoghParmar]

@MarkEWaite Thanks for the review and the helpful guidance.

I have now:

• Restored the full pull request template, including the submitter and maintainer checklists

• Added the changelog entry and category

• Updated the Pipeline example to match what the declarative directive generator suggests

Please let me know if anything else should be adjusted.

[MarkEWaite]

@MarkEWaite Thanks for the review and the helpful guidance.

I have now:

• Restored the full pull request template, including the submitter and maintainer checklists
• Added the changelog entry and category
• Updated the Pipeline example to match what the declarative directive generator suggests

Please let me know if anything else should be adjusted.

You are sabotaging yourself by not doing local checks and by not reviewing the results of the automated checks performed on the pull request. Those automated checks fail the build due to a formatting error. The contributing guide tells you how to correct the formatting error.

• Build the change locally by following the instructions in the contributing guide.
  It doesn't appear that you read the contributing guide section that discusses how to handle code formatting and didn't apply the formatting fixes
• Use the pull request template and provide all the information it requests.
  You didn't include all the items when you re-added the maintainer checklist and you didn't actually address the question in the submitter checklist that asks for an explanation why automated tests are not included. I agree that this change does not need automated tests, but you didn't address the checklist item in your pull request. Please state that you intentionally did not add an automated test.

While reviewing your change locally, I discovered that my original change proposal was wrong. I used the <code> HTML tag to surround the example Pipeline script. That was incorrect. The Jenkins online help surrounds examples with the <pre> HTML tag. While working with yarn lint:fix, I found that the formatting needed to be adjusted. I made those two corrections and pushed the change.

[MarkEWaite]

Now that you've developed experience adding online help to Jenkins, it would be nice if you could submit another pull request (or two) that add online help to the other parts of the build discarder. Don't do that in this pull request. It is ready to merge as far as I can tell.

[MarkEWaite]

This PR is now ready for merge. We will merge it after approximately 24 hours if there is no negative feedback.

/label ready-for-merge