wxTimer::Start() only works from the main thread: mention it in the docs and add an assert in the code

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_2_4_BRANCH@17475 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

docs/latex/wx/timer.tex

@@ -20,6 +20,8 @@ In any case, you must start the timer with \helpref{Start}{wxtimerstart}
 after constructing it before it actually starts sending notifications. It can
 be stopped later with \helpref{Stop}{wxtimerstop}.
 
+{\bf NB:} note that timer can only be used from the main thread currently.
+
 \wxheading{Derived from}
 
 \helpref{wxObject}{wxobject}