Expand wxString overview and document some problems due to its dual nature.

Explain the possible problems with wxString due to its dual Unicode/ASCII
nature.

Also document the various conversions in the overview itself.

Closes #14694.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@73905 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775
Mention using wxString::c_str() with vararg functions in the change log.

This is sufficiently important to be mentioned here in addition to the main
documentation. Especially considering that this results in run-time crashes
instead of (just) compile-time errors with MSVC.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@73906 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775