Enhance age dataset documentation

[rly]

Expanded documentation for the 'age' dataset, including details on ISO 8601 Duration format and age range representation.

Copied from NWB best practices nwbinspector.readthedocs.io/en/dev/best_practices/nwbfile_metadata.html#subject

In concert with NeurodataWithoutBorders/pynwb#2161

Checklist

For all schema changes:

• Add release notes for the PR to docs/format/source/format_release_notes.rst.
• Have you included the relevant issue number using "Fix #XXX" notation where XXX is the issue number? By including "Fix #XXX" you allow GitHub to close issue #XXX when the PR is merged.
• Make sure that hdmf-common-schema points to the latest release and not the latest commit on the main branch.

[h-mayorquin]

Thanks for this.