QQuaternion: comment all #endif's

It's easier to comprehend what's going on, because there are quite a
few different ones here.

Exception: if the #if block contains only very few lines (max four or
so, didn't count), then a naked #endif is ok, and arguably easier on
the eye, so leave them in that case.

Amends the start of the public history.

Pick-to: 6.5
Change-Id: Ib1b27859344916caa17dc1ff0010b6b10d34bb13
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>
(cherry picked from commit d9e77680600de1bdcd8009c6a2cfdf82de2085fc)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>
(cherry picked from commit 1cf2fffee41fdb00deb52e2f330a9fca10e95734)
(cherry picked from commit 595c9956d59a8a57b729efbb22d442787bccc4e9)

src/gui/math3d/qquaternion.h

@@ -128,7 +128,7 @@ QT_WARNING_POP
 
     QT7_ONLY(Q_GUI_EXPORT) static QQuaternion rotationTo(const QVector3D &from,
                                                          const QVector3D &to);
-#endif
+#endif // QT_NO_VECTOR3D
 
     QT7_ONLY(Q_GUI_EXPORT) static QQuaternion slerp(const QQuaternion &q1, const QQuaternion &q2,
                                                     float t);
@@ -330,7 +330,7 @@ inline QQuaternion QQuaternion::fromEulerAngles(const QVector3D &eulerAngles)
     return QQuaternion::fromEulerAngles(eulerAngles.x(), eulerAngles.y(), eulerAngles.z());
 }
 
-#endif
+#endif // QT_NO_VECTOR3D
 
 constexpr void QQuaternion::setVector(float aX, float aY, float aZ) noexcept
 {
@@ -349,7 +349,7 @@ constexpr QVector4D QQuaternion::toVector4D() const noexcept
     return QVector4D(xp, yp, zp, wp);
 }
 
-#endif
+#endif // QT_NO_VECTOR4D
 
 #ifndef QT_NO_DEBUG_STREAM
 Q_GUI_EXPORT QDebug operator<<(QDebug dbg, const QQuaternion &q);
@@ -360,8 +360,8 @@ Q_GUI_EXPORT QDataStream &operator<<(QDataStream &, const QQuaternion &);
 Q_GUI_EXPORT QDataStream &operator>>(QDataStream &, QQuaternion &);
 #endif
 
-#endif
+#endif // QT_NO_QUATERNION
 
 QT_END_NAMESPACE
 
-#endif
+#endif // QQUATERNION_H