Make wxDateTime::Tm::yday public and document it.

Make wxDateTime::Tm::yday public and document it.

There doesn't seem any reason to allow access to all the other struct Tm
fields but not yday so make it public, fill it in correctly when creating Tm
without using its ctor from struct tm and document struct Tm itself including
its yday field.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@65649 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775