As of right now, https://doc.qt.io/qt-5/qobject.html#tr shows a signature for QObject::tr as

QString QObject::tr(const char *sourceText, const char *disambiguation = Q_OBJECT, int n = Q_OBJECT)

The source for the documentation in qobject.cpp is

{{/*!}}
  \fn QString QObject::tr(const char *sourceText, const char *disambiguation, int n)
  \reentrant

and the signature in the header is

static QString tr(const char *sourceText, const char * = nullptr, int = -1)

qdoc should document the default values of the parameters. Note that the parameters are unnamed in the header file, which might be throwing qdoc off somehow.