Enhance 'age' parameter documentation

[rly]

Expanded documentation for the 'age' parameter to include age range examples and clarify usage.

Copied from https://nwbinspector.readthedocs.io/en/dev/best_practices/nwbfile_metadata.html#subject

Motivation

I received a question in a direct message: "I had a question about the age requirement for publishing to DANDI using NWB. If the user does not know the exact date of birth, can the user provide age as "3-8 months old" for the age of the subject during a given experiment? I've asked the researcher and he said that he doesn't record the date of birth since he doesn't view it as relevant for his experiments."

This could be better documented in pynwb, so I am adding that here. I will submit a separate PR for improving the schema doc.

How to test the behavior?

View API docs

Checklist

• Did you update CHANGELOG.md with your changes?
• Have you checked our Contributing document?
• Have you ensured the PR clearly describes the problem and the solution?
• Is your contribution compliant with our coding style? This can be checked running ruff check . && codespell from the source directory.
• Have you checked to ensure that there aren't other open Pull Requests for the same change?
• 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.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.23%. Comparing base (4b7f551) to head (cb234bd).

Additional details and impacted files

@@           Coverage Diff           @@
##              dev    #2161   +/-   ##
=======================================
  Coverage   95.23%   95.23%           
=======================================
  Files          29       29           
  Lines        2876     2876           
  Branches      740      740           
=======================================
  Hits         2739     2739           
  Misses         83       83           
  Partials       54       54           

Flag	Coverage Δ
integration	74.44% <0.00%> (ø)
unit	84.94% <100.00%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.