Remove TODO statement from QWinRegistryKey::stringValue()

QWinRegistryKey::stringValue is more readable than
key::value<QString>().value_or(QString()).

This patch also adds developer documentation for the function.

Change-Id: I6a760db77a50ca9fa810196e14af14068df8aee3
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
(cherry picked from commit 16ca13e148445b5a18ea768c096dfedfbacce3c4)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/corelib/kernel/qwinregistry.cpp

@@ -120,6 +120,10 @@ QVariant QWinRegistryKey::value(QStringView subKey) const
     return {};
 }
 
+// Returns the value of the specified subKey as a string, obtained using
+// qvariant_cast from the underlying QVariant. If that value is not a string,
+// or the subKey has no value, a string whose isNull() is true is returned.
+// Otherwise, the resulting string (which may be empty) is returned.
 QString QWinRegistryKey::stringValue(QStringView subKey) const
 {
     return value<QString>(subKey).value_or(QString());

src/corelib/kernel/qwinregistry_p.h

@@ -56,7 +56,6 @@ public:
         return std::nullopt;
     }
 
-    // ### TODO: Remove once all usages are migrated to new interface.
     QString stringValue(QStringView subKey) const;
 
 private: