Doc: fix documentation warning for QByteArray::fromCFData

\brief tags are not used for member function documentation.
Explicitly set the prototype for qdoc instead, it doesn't seem to
know what to do with the definition in an ObjectiveC file and
produces link warnings.

Pick-to: 6.9 6.9.0
Change-Id: I89252e7b84586e987ed6d950d90142e916d8f1df
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>

Author: vohi

src/corelib/kernel/qcore_foundation.mm

@@ -26,7 +26,9 @@
 QT_BEGIN_NAMESPACE
 
 /*!
-    \brief Constructs a new QByteArray containing a copy of the CFData \a data.
+    \fn QByteArray QByteArray::fromCFData(CFDataRef data)
+
+    Constructs a new QByteArray containing a copy of the CFData \a data.
 
     \since 5.3
     \ingroup platform-type-conversions