Mention wxLocale in wxNumValidator documentation.

Using setlocale() can thoroughly confuse wxNumValidator because it results in
a mismatch between the decimal and/or thousands separators it uses, returned
by wxLocale, and the actual separators in the strings, formatted by the CRT.

So mention in the documentation that using setlocale() is a bad idea.

Closes #12970.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@68863 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775