Unify QLoggingCategory's two constructors

This follows upon commit ca0df4b2694fc3ca0c31cda0d5edec7e76f7dfcb's
comment requesting the unification. Tidied docs at the same time.

Task-number: QTBUG-85700
Change-Id: Ia12dbe98b94388030c7a06036c5287cfbcc4fd05
Reviewed-by: Kai Koehne <kai.koehne@qt.io>

src/corelib/io/qloggingcategory.cpp

@@ -196,28 +196,15 @@ static void setBoolLane(QBasicAtomicInt *atomic, bool enable, int shift)
     \snippet qloggingcategory/main.cpp 3
 */
 
-/*!
-    Constructs a QLoggingCategory object with the provided \a category name.
-    All message types for this category are enabled by default.
-
-    If \a category is \c{0}, the category name is changed to \c "default".
-
-    \note \a category must be kept valid during the lifetime of this object.
-*/
-QLoggingCategory::QLoggingCategory(const char *category)
-    : d(nullptr),
-      name(nullptr)
-{
-    init(category, QtDebugMsg);
-}
-
 /*!
     Constructs a QLoggingCategory object with the provided \a category name,
-    and enables all messages with types more severe or equal than \a enableForLevel.
+    and enables all messages with types at least as verbose as \a enableForLevel,
+    which defaults to QtDebugMsg (which enables all categories).
 
-    If \a category is \c{0}, the category name is changed to \c "default".
+    If \a category is \nullptr, the category name \c "default" is used.
 
     \note \a category must be kept valid during the lifetime of this object.
+    Using a string literal for it is the usual way to achieve this.
 
     \since 5.4
 */

src/corelib/io/qloggingcategory.h

@@ -49,9 +49,7 @@ class Q_CORE_EXPORT QLoggingCategory
 {
     Q_DISABLE_COPY(QLoggingCategory)
 public:
-    // ### Qt 6: Merge constructors
+    explicit QLoggingCategory(const char *category, QtMsgType severityLevel = QtDebugMsg);
-    explicit QLoggingCategory(const char *category);
-    QLoggingCategory(const char *category, QtMsgType severityLevel);
     ~QLoggingCategory();
 
     bool isEnabled(QtMsgType type) const;