in_tail: stale ignore_older description still says parser/record-level behavior

The `in_tail` config description for `ignore_older` does not match current behavior.

[Current text in plugins/in_tail/tail.c](https://github.com/fluent/fluent-bit/blob/cb7256c2867494ae404ab6e3da936e8f6e0c9c94/plugins/in_tail/tail.c#L655-L662) says `ignore_older` ignores records and is only available when a parser is configured.

However, since [ee77895](https://github.com/fluent/fluent-bit/commit/ee77895) (Feb 10, 2021), `ignore_older` was moved to file modification time filtering (file-level semantics), so this wording is no longer accurate. The commit [42029a](https://github.com/fluent/fluent-bit/commit/42029a4f50536db12e6a4e62562aae1ddad61fe9) (Mar 15, 2024) appears to have reintroduced text about the old behavior.

I found this issue from 2020 that has the old description verbatim ([issue #2466](https://github.com/fluent/fluent-bit/issues/2466)).

## Expected behavior
`ignore_older` description should reflect file mtime-based filtering that it does today.

## Actual behavior
Config/help text still describes old parser/record semantics.

## Proposed fix
Update comments/docs in plugins/in_tail/tail.c (and related inline comments, e.g. tail_config.h) to describe file-level age filtering.

Read the `ignore_older` description in `plugins/in_tail/tail.c` around lines 655-662 and the related inline comments in `tail_config.h`. Compare the wording with the current file modification-time filtering behavior described in the issue. Done means the config and inline help text consistently describe file-level age filtering without parser or record-level claims.

Written by the indexing model from the issue text.