docs(input[week]): clarify value of Date object

Add a note to documentation of input[week] to explicitly state that the
resulting Date object's value is set to Thursday at midnight of the
specified week.

Resolves #15883 
Closes #16104

Author: SteveAndrewArcher

src/ng/directive/input.js

@@ -457,6 +457,10 @@ var inputType = {
     * The model must always be a Date object, otherwise AngularJS will throw an error.
     * Invalid `Date` objects (dates whose `getTime()` is `NaN`) will be rendered as an empty string.
     *
+    * The value of the resulting Date object will be set to Thursday at 00:00:00 of the requested week,
+    * due to ISO-8601 week numbering standards. Information on ISO's system for numbering the weeks of the
+    * year can be found at: https://en.wikipedia.org/wiki/ISO_8601#Week_dates
+    *
     * The timezone to be used to read/write the `Date` instance in the model can be defined using
     * {@link ng.directive:ngModelOptions ngModelOptions}. By default, this is the timezone of the browser.
     *