docs: add note for datepicker/datepicker-popup

* update the datepicker and datepicker popup docs to let user know
  about using the `uibDateParser` service to convert a date string
  into a `Date` object.

Closes #5880

src/datepicker/docs/readme.md

@@ -13,7 +13,7 @@ The datepicker has 3 modes:
 * `ng-model`
   <small class="badge">$</small>
   <i class="glyphicon glyphicon-eye-open"></i> -
-  The date object. Needs to be a Javascript Date object.
+  The date object. Must be a Javascript `Date` object. You may use the `uibDateParser` service to assist in string-to-object conversion.
 
 * `ng-model-options`
   <small class="badge">$</small>

src/datepickerPopup/docs/readme.md

@@ -57,6 +57,11 @@ Add the ability to override the template used on the component (inner uib-datepi
 _(Default: `false`)_ -
 Whether or not to show the datepicker.
 
+* `ng-model`
+  <small class="badge">$</small>
+  <i class="glyphicon glyphicon-eye-open"></i> -
+  The date object. Must be a Javascript `Date` object. You may use the `uibDateParser` service to assist in string-to-object conversion.
+
 * `on-open-focus`
 <small class="badge">$</small>
 <small class="badge">C</small>