QDirListing: add API docs for DirEntry

It's part of the public API.

Drive-by change: add `\ingroup io` to QDirListing too.

Fixes: QTBUG-127152
Change-Id: Ic327cbb8742dfd323cae9f7135a7590d14819869
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
(cherry picked from commit 8fc89a4637214a0724a2abb5f47c16487d4fffea)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/corelib/io/qdirlisting.cpp

@@ -6,6 +6,7 @@
     \since 6.8
     \class QDirListing
     \inmodule QtCore
+    \ingroup  io
     \brief The QDirListing class provides an STL-style iterator for directory entries.
 
     You can use QDirListing to navigate entries of a directory one at a time.
@@ -718,6 +719,25 @@ QDirListing::const_iterator &QDirListing::const_iterator::operator++()
     return *this;
 }
 
+/*!
+    \class QDirListing::DirEntry
+    \inmodule QtCore
+    \ingroup  io
+
+    Dereferencing a valid QDirListing::const_iterator returns a DirEntry
+    object.
+
+    DirEntry offers a subset of QFileInfo's API (for example, fileName(),
+    filePath(), exists()). Internally, DirEntry only constructs a QFileInfo
+    object if needed, that is, if the info hasn't been already fetched
+    by other system functions. You can use DirEntry::fileInfo() to get a
+    QFileInfo. For example:
+
+    \snippet code/src_corelib_io_qdirlisting.cpp 3
+
+    \snippet code/src_corelib_io_qdirlisting.cpp 4
+*/
+
 /*!
     \fn QFileInfo QDirListing::DirEntry::fileInfo() const
     \fn QString QDirListing::DirEntry::fileName() const