doc(Duration): extend unit accessors documentation

* include reference to different behaviour for Durations created out of factory methods

Author: KlausNie

src/duration.js

@@ -864,6 +864,11 @@ export default class Duration {
 
   /**
    * Get the years.
+   *
+   * Note: Durations created out of Duration.from* (e.g. fromObject) factories will always yield 0.
+   * This will give you the correct difference in years, if the Duration was created out of a diff operation.
+   *
+   * More information: https://moment.github.io/luxon/#/math?id=duration-math
    * @type {number}
    */
   get years() {
@@ -872,6 +877,11 @@ export default class Duration {
 
   /**
    * Get the quarters.
+   *
+   * Note: Durations created out of Duration.from* (e.g. fromObject) factories will always yield 0.
+   * This will give you the correct difference in years, if the Duration was created out of a diff operation.
+   *
+   * More information: https://moment.github.io/luxon/#/math?id=duration-math
    * @type {number}
    */
   get quarters() {
@@ -880,6 +890,11 @@ export default class Duration {
 
   /**
    * Get the months.
+   *
+   * Note: Durations created out of Duration.from* (e.g. fromObject) factories will always yield 0.
+   * This will give you the correct difference in years, if the Duration was created out of a diff operation.
+   *
+   * More information: https://moment.github.io/luxon/#/math?id=duration-math
    * @type {number}
    */
   get months() {
@@ -888,6 +903,11 @@ export default class Duration {
 
   /**
    * Get the weeks
+   *
+   * Note: Durations created out of Duration.from* (e.g. fromObject) factories will always yield 0.
+   * This will give you the correct difference in years, if the Duration was created out of a diff operation.
+   *
+   * More information: https://moment.github.io/luxon/#/math?id=duration-math
    * @type {number}
    */
   get weeks() {
@@ -896,6 +916,11 @@ export default class Duration {
 
   /**
    * Get the days.
+   *
+   * Note: Durations created out of Duration.from* (e.g. fromObject) factories will always yield 0.
+   * This will give you the correct difference in years, if the Duration was created out of a diff operation.
+   *
+   * More information: https://moment.github.io/luxon/#/math?id=duration-math
    * @type {number}
    */
   get days() {
@@ -904,6 +929,11 @@ export default class Duration {
 
   /**
    * Get the hours.
+   *
+   * Note: Durations created out of Duration.from* (e.g. fromObject) factories will always yield 0.
+   * This will give you the correct difference in years, if the Duration was created out of a diff operation.
+   *
+   * More information: https://moment.github.io/luxon/#/math?id=duration-math
    * @type {number}
    */
   get hours() {
@@ -912,6 +942,11 @@ export default class Duration {
 
   /**
    * Get the minutes.
+   *
+   * Note: Durations created out of Duration.from* (e.g. fromObject) factories will always yield 0.
+   * This will give you the correct difference in years, if the Duration was created out of a diff operation.
+   *
+   * More information: https://moment.github.io/luxon/#/math?id=duration-math
    * @type {number}
    */
   get minutes() {
@@ -920,6 +955,11 @@ export default class Duration {
 
   /**
    * Get the seconds.
+   *
+   * Note: Durations created out of Duration.from* (e.g. fromObject) factories will always yield 0.
+   * This will give you the correct difference in years, if the Duration was created out of a diff operation.
+   *
+   * More information: https://moment.github.io/luxon/#/math?id=duration-math
    * @return {number}
    */
   get seconds() {
@@ -928,6 +968,11 @@ export default class Duration {
 
   /**
    * Get the milliseconds.
+   *
+   * Note: Durations created out of Duration.from* (e.g. fromObject) factories will always yield 0.
+   * This will give you the correct difference in years, if the Duration was created out of a diff operation.
+   *
+   * More information: https://moment.github.io/luxon/#/math?id=duration-math
    * @return {number}
    */
   get milliseconds() {
@@ -987,4 +1032,4 @@ export default class Duration {
     }
     return true;
   }
-}
+}