docs(utils): document missing methods

lib/utils/color_extension.dart

@@ -3,7 +3,29 @@ import 'dart:collection' show SplayTreeMap;
 import 'package:cabin_booking/utils/num_extension.dart';
 import 'package:flutter/material.dart';
 
+/// Color extension.
 extension ColorExtension on Color {
+  /// Returns a new threshold to color Map based on the number of [samples]
+  /// and up to the [highestValue] for the threshold.
+  ///
+  /// An optional [minOpacity] value can be provided for the first opacity
+  /// value in the Map.
+  ///
+  /// Throws an [ArgumentError] if [samples] is not greater than zero.
+  ///
+  /// Example:
+  /// ```dart
+  /// const color = Colors.blue;
+  /// const thresholds = {
+  ///   1: Color(0x332196f3),
+  ///   4: Color(0x552196f3),
+  ///   8: Color(0xaa2196f3),
+  ///   12: Color(0xff2196f3),
+  /// };
+  /// assert(
+  ///   color.opacityThresholds(highestValue: 12, samples: 3) == thresholds,
+  /// );
+  /// ```
   Map<int, Color> opacityThresholds({
     required int highestValue,
     required int samples,

lib/utils/map_extension.dart

@@ -1,4 +1,19 @@
+/// Map extension.
 extension MapExtension<K, V> on Map<K, V> {
+  /// Returns a new [Map] filled with the result of [ifAbsent] for [keys] that
+  /// are not present in this [Map].
+  ///
+  /// Unlike [Map.update], this method does not mutate this [Map].
+  ///
+  /// Example:
+  /// ```dart
+  /// const map = <int, bool?>{1: true, 2: false};
+  /// final filled = map.fillEmptyKeyValues(
+  ///   keys: const [0, 1, 2, 5],
+  ///   ifAbsent: () => null,
+  /// );
+  /// assert(filled == const {0: null, 1: true, 2: false, 5: null});
+  /// ```
   Map<K, V> fillEmptyKeyValues({
     Iterable<K> keys = const [],
     required V Function() ifAbsent,