Add a comment about NaN in min/max extensions (#234)

* Add a comment about the treatment of NaN in `min`/`max` extensions on number types.

lib/src/iterable_extensions.dart

@@ -592,6 +592,8 @@ extension IterableNullableExtension<T extends Object> on Iterable<T?> {
 /// since doubles require special handling of [double.nan].
 extension IterableNumberExtension on Iterable<num> {
   /// A minimal element of the iterable, or `null` it the iterable is empty.
+  ///
+  /// If any element is [NaN](double.nan), the result is NaN.
   num? get minOrNull {
     var iterator = this.iterator;
     if (iterator.moveNext()) {
@@ -615,10 +617,14 @@ extension IterableNumberExtension on Iterable<num> {
 
   /// A minimal element of the iterable.
   ///
+  /// If any element is [NaN](double.nan), the result is NaN.
+  ///
   /// The iterable must not be empty.
   num get min => minOrNull ?? (throw StateError('No element'));
 
   /// A maximal element of the iterable, or `null` if the iterable is empty.
+  ///
+  /// If any element is [NaN](double.nan), the result is NaN.
   num? get maxOrNull {
     var iterator = this.iterator;
     if (iterator.moveNext()) {
@@ -642,6 +648,8 @@ extension IterableNumberExtension on Iterable<num> {
 
   /// A maximal element of the iterable.
   ///
+  /// If any element is [NaN](double.nan), the result is NaN.
+  ///
   /// The iterable must not be empty.
   num get max => maxOrNull ?? (throw StateError('No element'));
 
@@ -765,6 +773,8 @@ extension IterableIntegerExtension on Iterable<int> {
 /// [IterableComparableExtension] since doubles are only `Comparable<num>`.
 extension IterableDoubleExtension on Iterable<double> {
   /// A minimal element of the iterable, or `null` it the iterable is empty.
+  ///
+  /// If any element is [NaN](double.nan), the result is NaN.
   double? get minOrNull {
     var iterator = this.iterator;
     if (iterator.moveNext()) {
@@ -788,10 +798,14 @@ extension IterableDoubleExtension on Iterable<double> {
 
   /// A minimal element of the iterable.
   ///
+  /// If any element is [NaN](double.nan), the result is NaN.
+  ///
   /// The iterable must not be empty.
   double get min => minOrNull ?? (throw StateError('No element'));
 
   /// A maximal element of the iterable, or `null` if the iterable is empty.
+  ///
+  /// If any element is [NaN](double.nan), the result is NaN.
   double? get maxOrNull {
     var iterator = this.iterator;
     if (iterator.moveNext()) {
@@ -815,6 +829,8 @@ extension IterableDoubleExtension on Iterable<double> {
 
   /// A maximal element of the iterable.
   ///
+  /// If any element is [NaN](double.nan), the result is NaN.
+  ///
   /// The iterable must not be empty.
   double get max => maxOrNull ?? (throw StateError('No element'));