Date and Time classes: use new \compares command

... to describe the comparison operators provided by these classes.

Do not remove the existing documentation of individual operators for
now, because the new commands do not contain any links to explain what
do the types of ordering actually mean.

Task-number: QTBUG-119433
Change-Id: I663b992377ea8b00ffa0c0a64da0733e3772d1dd
Reviewed-by: Marc Mutz <marc.mutz@qt.io>
(cherry picked from commit e766a338345a3ac00d9e6ae498353616dd5d4c77)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/corelib/time/qdatetime.cpp

@@ -348,6 +348,12 @@ static int fromOffsetString(QStringView offsetString, bool *valid) noexcept
     \reentrant
     \brief The QDate class provides date functions.
 
+    \compares strong
+    \compareswith strong std::chrono::year_month_day std::chrono::year_month_day_last \
+                  std::chrono::year_month_weekday std::chrono::year_month_weekday_last
+    These comparison operators are only available when using C++20.
+    \endcompareswith
+
     A QDate object represents a particular day, regardless of calendar, locale
     or other settings used when creating it or supplied by the system.  It can
     report the year, month and day of the month that represent the day with
@@ -1949,6 +1955,8 @@ bool QDate::isLeapYear(int y)
 
     \brief The QTime class provides clock time functions.
 
+    \compares strong
+
     A QTime object contains a clock time, which it can express as the numbers of
     hours, minutes, seconds, and milliseconds since midnight. It provides
     functions for comparing times and for manipulating a time by adding a number
@@ -3556,6 +3564,8 @@ QDateTime::Data QDateTimePrivate::create(QDate toDate, QTime toTime, const QTime
     \reentrant
     \brief The QDateTime class provides date and time functions.
 
+    \compares weak
+
     A QDateTime object encodes a calendar date and a clock time (a "datetime")
     in accordance with a time representation. It combines features of the QDate
     and QTime classes. It can read the current datetime from the system

src/corelib/time/qtimezone.cpp

@@ -95,6 +95,8 @@ Q_GLOBAL_STATIC(QTimeZoneSingleton, global_tz);
 
     \brief QTimeZone identifies how a time representation relates to UTC.
 
+    \compares equality
+
     When dates and times are combined, the meaning of the result depends on how
     time is being represented. There are various international standards for
     representing time; one of these, UTC, corresponds to the traditional