Fix QCalendar::isValid(): make it const

Spotted in API change review, thanks to Albert Astals Cid.
Also added documentation of this method.

Change-Id: I2ef2c526a98b571a3cb3bb5f93d1952b1b0d63a9
Reviewed-by: Albert Astals Cid <albert.astals.cid@kdab.com>

src/corelib/time/qcalendar.cpp

@@ -701,7 +701,7 @@ const QCalendarBackend *QCalendarBackend::fromEnum(QCalendar::System system)
     calendar being constructed by other means first. With no argument, the
     default constructor returns the Gregorian calendar.
 
-    \sa QCalendar, System
+    \sa QCalendar, System, isValid()
 */
 
 QCalendar::QCalendar()
@@ -723,6 +723,15 @@ QCalendar::QCalendar(QLatin1String name)
 QCalendar::QCalendar(QStringView name)
     : d(QCalendarBackend::fromName(name)) {}
 
+/*
+    \fn bool QCalendar::isValid() const
+
+    Returns true if this is a valid calendar object.
+
+    Constructing a calendar with an unrecognised calendar name may result in an
+    invalid object. Use this method to check after creating a calendar by name.
+*/
+
 // Date queries:
 
 /*!

src/corelib/time/qcalendar.h

@@ -137,7 +137,7 @@ public:
     explicit QCalendar(QStringView name);
 
     // QCalendar is a trivially copyable value type.
-    bool isValid() { return d != nullptr; }
+    bool isValid() const { return d != nullptr; }
 
     // Date queries:
     int daysInMonth(int month, int year = Unspecified) const;