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

include/wx/textctrl.h

@@ -734,6 +734,9 @@ public:
        wxTextEntry::SetValue(value);
     }
 
+    // wxTextEntry overrides
+    virtual bool SetHint(const wxString& hint);
+
     // wxWindow overrides
     virtual wxVisualAttributes GetDefaultAttributes() const
     {