Enhance age dataset documentation (#669)

Author: rly

core/nwb.file.yaml

@@ -415,7 +415,12 @@ groups:
   datasets:
   - name: age
     dtype: text
-    doc: Age of subject. Can be supplied instead of 'date_of_birth'.
+    doc: "Age of subject. Can be supplied instead of 'date_of_birth'.
+      The ISO 8601 Duration format is recommended, e.g., 'P90D' for 90 days old.
+      If the precise age is unknown, an age range can be given by '[lower bound]/[upper bound]' e.g.
+      'P10D/P20D' would mean that the age is in between 10 and 20 days. If only the lower bound is known,
+      then including only the slash after that lower bound can be used to indicate a missing bound.
+      For instance, 'P90Y/' would indicate that the age is 90 years or older."
     quantity: '?'
     attributes:
       - name: reference

docs/format/source/format_release_notes.rst

@@ -12,6 +12,8 @@ Minor changes
   that are typically used together for analysis, such as spike sorting. (#659)
 - Specified that units for ``ElectrodesTable`` coordinate fields (``x``, ``y``, ``z``, ``rel_x``, ``rel_y``, ``rel_z``)
   should be in microns. (#658)
+- Expanded documentation for the ``Subject`` 'age' dataset, including details on ISO 8601 Duration format and 
+  age range representation.
 
 
 2.9.0 (June 26, 2025)