Clarify what time-zone "transitions" are.

Change-Id: Idad5b841c3c693e2040ca606894187988615c9b0
Reviewed-by: Topi Reiniö <topi.reinio@theqtcompany.com>

src/corelib/tools/qtimezone.cpp

@@ -695,6 +695,11 @@ QTimeZone::OffsetData QTimeZone::offsetData(const QDateTime &forDateTime) const
 
 /*!
     Returns \c true if the system backend supports obtaining transitions.
+
+    Transitions are changes in the time-zone: these happen when DST turns on or
+    off and when authorities alter the offsets for the time-zone.
+
+    \sa nextTransition(), previousTransition(), transitions()
 */
 
 bool QTimeZone::hasTransitions() const