MyClass *mc = MyClass::instance();
qDebug() << mc->sampleFunc(1);

myclass.h:

#ifndef MYCLASS_H
#define MYCLASS_H

#include <QtCore>
#include <QObject>
#include <QQuickItem>

class MyClass : public QObject
{
    Q_OBJECT
    Q_DISABLE_COPY(MyClass)
    QML_SINGLETON
public:
    explicit MyClass(QObject *parent = nullptr);
    static MyClass *instance();
    static QObject* qmlInstance(QQmlEngine *engine, QJSEngine* scriptEngine);
    void registerTypes() {
        qmlRegisterSingletonType<MyClass>("MyClass",1,0,"MyClass",&MyClass::qmlInstance);
    }
    qreal sampleFunc(qreal input);
private:
    static MyClass *m_pThis;
};

#endif // MYCLASS_H

myclass.cpp:

#include "myclass.h"

MyClass *MyClass::m_pThis = nullptr;
MyClass *MyClass::instance() {
    if(m_pThis == nullptr) {
        m_pThis = new MyClass;
    }
    return m_pThis;
}

MyClass::MyClass(QObject *parent)
    : QObject(parent)
{
    registerTypes();
}
/*!
 * \brief MyClass::sampleFunc.
 * \fn qreal MyClass::sampleFunc(qreal input)
 * \a input
 */
qreal MyClass::sampleFunc(qreal input) {
    return input;
}

MyClass.qdoc:

/*!
  \class MyClass
  \inmodule MyClass
  \title MyClass
  */ 

MyClass.qdocconf:

project = MyClass
description = MyClass Doc

sourcedirs = .
headerdirs = .

sources.fileextensions = "*.cpp *.qdoc *.mm *.qml"
headers.fileextensions = "*.h *.ch *.h++ *.hh *.hpp *.hxx"


outputdir  =    ./html
outputformats = HTML

Can you share the header and code of that class ?
From the looks of it, it seems to not find the implementation of the function.