Doc: Hide QTextInlineObject constructor from the documentation

The constructor takes an internal class instance as a parameter,
and is not meant to be used externally. Add a note in the
description and hide the constructor.

Task-number: QTBUG-39785
Change-Id: Ice850f0968880f3933ebb7ae83efeaeed85441cb
Reviewed-by: Konstantin Ritt <ritt.ks@gmail.com>
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@digia.com>

src/gui/text/qtextlayout.cpp

@@ -108,6 +108,10 @@ QT_BEGIN_NAMESPACE
     This class is only used if the text layout is used to lay out
     parts of a QTextDocument.
 
+    Normally, you do not need to create a QTextInlineObject. It is
+    used by QAbstractTextDocumentLayout to handle inline objects when
+    implementing a custom layout.
+
     The inline object has various attributes that can be set, for
     example using, setWidth(), setAscent(), and setDescent(). The
     rectangle it occupies is given by rect(), and its direction by
@@ -117,6 +121,7 @@ QT_BEGIN_NAMESPACE
 
 /*!
     \fn QTextInlineObject::QTextInlineObject(int i, QTextEngine *e)
+    \internal
 
     Creates a new inline object for the item at position \a i in the
     text engine \a e.