Fix QMutex documentation saying some function are static while they are not

qdoc only see a fake QMutex class (the same as the one built in bootstrap)
But that fake QMutex had static member while the normal QMutex class
has non static member.

QMutexLocker::mutex is also a const function in the real QMutexLocker

Task-number: QTBUG-38522
Change-Id: I220434ffc6a9e990029f770e2536ecb55b4e2182
Reviewed-by: Martin Smith <martin.smith@digia.com>
Reviewed-by: Jędrzej Nowacki <jedrzej.nowacki@digia.com>
Add a comment stating QMutex::isRecursive should be made const in Qt6

Change-Id: I452b0764790112c59af77bc8d95f403ff37cbc4a
Reviewed-by: Jędrzej Nowacki <jedrzej.nowacki@digia.com>