Forbid setting hints for multi-line text controls.

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