docs: clarify IsUtcOffset accepts Z/+00:00/-00:00; trim default-value comment

Addresses review feedback: document that any zero-offset spelling (Z, +00:00,
-00:00) is accepted by TemporalUtils::IsUtcOffset, and shorten the verbose
comment on the default-value members.

Author: huan233usc

src/iceberg/schema_field.h

@@ -134,9 +134,7 @@ class ICEBERG_EXPORT SchemaField : public iceberg::util::Formattable {
   std::shared_ptr<Type> type_;
   bool optional_;
   std::string doc_;
-  // Default values are owned by this field and never mutated after being set; copies
-  // of the field share the same payload (reference-counted) instead of deep-copying,
-  // like `type_` above. Sharing is unobservable because the payload is immutable.
+  // Immutable default values, shared (not deep-copied) across field copies, like `type_`.
   std::shared_ptr<const Literal> initial_default_;
   std::shared_ptr<const Literal> write_default_;
 };

src/iceberg/util/temporal_util.h

@@ -125,15 +125,19 @@ class ICEBERG_EXPORT TemporalUtils {
   /// \return The number of nanoseconds since epoch (UTC), or an error.
   static Result<int64_t> ParseTimestampNsWithZone(std::string_view str);
 
-  /// \brief Reports whether a timestamp-with-zone string uses a UTC offset.
+  /// \brief Reports whether a timestamp-with-zone string carries a zero (UTC) offset.
   ///
   /// The ParseTimestamp*WithZone parsers accept any offset and silently normalize it
   /// to UTC. The spec's JSON single-value form for `timestamptz` / `timestamptz_ns`
-  /// default values only permits UTC ("Z" or "+00:00"), so callers that must enforce
-  /// that rule check the offset here before parsing.
+  /// default values is meant to be UTC, so callers that must enforce that rule check
+  /// the offset here before parsing.
+  ///
+  /// Any zero-offset spelling is treated as UTC: "Z", "+00:00" and "-00:00" all denote
+  /// a zero offset and are accepted, even though the spec writes the canonical form as
+  /// "+00:00". Only a genuinely non-zero offset (e.g. "+05:00") is rejected.
   ///
   /// \param str The timestamp-with-zone string to inspect.
-  /// \return true if the offset is UTC, false if it is a non-UTC offset, or an error
+  /// \return true if the offset is zero, false if it is a non-zero offset, or an error
   ///         if the timezone suffix cannot be parsed.
   static Result<bool> IsUtcOffset(std::string_view str);