docs(datepicker): Add example for setLocale method (#6224)

* docs(datepicker): Add example for setLocale method

Add a usage example to show how the formatting locale of the datepicker can be be changed as that wasn't documented before.

* docs(datepicker): Wording change for setLocale example

src/lib/datepicker/datepicker.md

@@ -128,6 +128,22 @@ By default the datepicker will use the locale code from the `LOCALE_ID` injectio
 export class MyApp {}
 ```
 
+It's also possible to set the locale at runtime using the `setLocale` method of the `DateAdapter`.
+
+```ts
+import { DateAdapter, NativeDateAdapter } from '@angular/material';
+
+@Component({
+  selector:    'foo',
+  template: ''
+})
+export class FooComponent {
+  constructor(dateAdapter: DateAdapter<NativeDateAdapter>) {
+    dateAdapter.setLocale('de-DE');
+  }
+}
+```
+
 #### Choosing a date implementation and date format settings
 The datepicker was built to be date implementation agnostic. This means that it can be made to work
 with a variety of different date implementations. However it also means that developers need to make