in_tail: fix ignore_older config description

[kai-wei-mo]

Update some comments and docs. Motivation is described in #12155.

Summary by CodeRabbit

• Documentation
  • Clarified that ignore_older filters files based on modification time.
  • Documented supported duration formats, including minutes, hours, and days.
  • Clarified that all files are read by default when the option is not set.

[coderabbitai]

📝 Walkthrough

Walkthrough

The tail plugin’s ignore_older documentation now describes file modification-time filtering, supported duration syntax, and the default behavior of reading all files.

Changes

Tail plugin documentation

Layer / File(s)	Summary
Correct ignore_older semantics plugins/in_tail/tail.c, plugins/in_tail/tail_config.h	Configuration help text and inline comments now refer to files and modification times, including m/h/d syntax and the default of reading all files.

Estimated code review effort: 1 (Trivial) | ~2 minutes

Possibly related issues

• fluent/fluent-bit#12155 — Corrects the ignore_older documentation from parser/record-level semantics to file modification-time filtering.

Suggested reviewers: cosmo0920

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title accurately summarizes the documentation-only change to the in_tail ignore_older config description.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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