Forbid setting hints for multi-line text controls.

This doesn't work anyhow, so make it explicit by asserting if an attempt to do
it is made. And document this.

Also add a test of SetHint() to the text sample to verify it still works.

Closes #14456.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@72010 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

interface/wx/textentry.h

@@ -456,6 +456,10 @@ public:
             currently you should avoid calling methods such as WriteText() or
             Replace() when using hints and the text control is empty.
 
+        @remarks Hints can only be used for single line text controls,
+            native multi-line text controls don't support hints under any
+            platform and hence wxWidgets doesn't provide them neither.
+
         @since 2.9.0
      */
     virtual bool SetHint(const wxString& hint);