Doc: Mention QDirIterator, QLockFile in IO overview page

Pick-to: 6.7
Change-Id: I013fc221ea790b0f3bbc41ef6bf3f63c643cdab6
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
(cherry picked from commit 0e26a2d74a5117d1a6b25f5038ccb55cffb58ade)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/corelib/io/qdiriterator.cpp

@@ -5,6 +5,7 @@
     \since 4.3
     \class QDirIterator
     \inmodule QtCore
+    \ingroup io
     \brief The QDirIterator class provides an iterator for directory entrylists.
 
     You can use QDirIterator to navigate entries of a directory one at a time.

src/corelib/io/qlockfile.cpp

@@ -42,6 +42,7 @@ static QString machineName()
 /*!
     \class QLockFile
     \inmodule QtCore
+    \ingroup io
     \brief The QLockFile class provides locking between processes using a file.
     \since 5.1