Doc: Add docs for rvalue references and move constructors

These members were introduced in 4.8, but left undocumented. Because we
consider undocumented API to be internal, the members are \since 5.2.

Change-Id: I52e2840a8cfaa7f59f410b3e2a06c0942ea06539
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>
Reviewed-by: Stephen Kelly <stephen.kelly@kdab.com>
QDateTime - Fix round-trip of second occurrence times

At the Daylight Tme to Standard Time transition, the local time repeats
itself, i.e. 2am occurs twice. Qt's behavior when setting this using
the local time is ambiguous, as it depends on the system implementation
of mktime, which behaves differently on different platforms.  Currently
this behavior remains undefined.  When setting using an msecs or time_t
value however we can determine the correct instance to use and cache it
to ensure that any conversion back from local time to msecs is performed
consistantly on all platforms.

Note that caching this value will result in any calculations being wrong
should the system time zone change, or its rules change.  This will be
fixed in Qt 5.3 when the system time zone change signal is implemented
and QDateTime switches to using QTimeZone instead of mktime to provide
consistnt behavior across platforms.

The QTimeZone spec does not require this fix as it already caches the
correct offset in setMSecsFromEpoch().

Change-Id: I799588db474e744a6d81e80f6a0442920569ebd3
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>