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

include/wx/datetime.h

@@ -462,7 +462,9 @@ public:
         //     instead of modifying the member fields directly!
     struct WXDLLIMPEXP_BASE Tm
     {
-        wxDateTime_t msec, sec, min, hour, mday;
+        wxDateTime_t msec, sec, min, hour,
+                     mday,  // Day of the month in 1..31 range.
+                     yday;  // Day of the year in 0..365 range.
         Month mon;
         int year;
 
@@ -497,9 +499,10 @@ public:
         // the timezone we correspond to
         TimeZone m_tz;
 
-        // these values can't be accessed directly because they're not always
-        // computed and we calculate them on demand
-        wxDateTime_t wday, yday;
+        // This value can only be accessed via GetWeekDay() and not directly
+        // because it's not always computed when creating this object and may
+        // need to be calculated on demand.
+        wxDateTime_t wday;
     };
 
     // static methods