Doc: Document the specific headers for the Qt Concurrent module

This commit makes the Image Scaling example use the optimized header
and updates documentation to include optimized header inclusion for
QtConcurrent functions.

Fixes: QTBUG-114663
Pick-to: 6.10 6.9 6.8
Change-Id: I50e4137ea3bc22bf88a3c9e4856043ba3d02f06e
Reviewed-by: Ivan Solovev <ivan.solovev@qt.io>

examples/qtconcurrent/imagescaling/imagescaling.h

@@ -4,7 +4,7 @@
 #define IMAGESCALING_H
 
 #include <QtWidgets>
-#include <QtConcurrent>
+#include <QtConcurrentRun>
 #include <QNetworkAccessManager>
 #include <optional>
 

src/concurrent/qtconcurrentfilter.cpp

@@ -26,6 +26,20 @@
     Note that the result types above are not QFuture objects, but real result
     types (in this case, QStringList and QSet<QString>).
 
+    \section1 Optimize includes
+
+    If you include the \c <QtConcurrent> header, the entire Qt Concurrent
+    module with the entire Qt Core module will be included, which may increase
+    compilation times and binary sizes. To use the
+    \l {QtConcurrent::filter}{QtConcurrent::filter()},
+    \l {QtConcurrent::filtered}{QtConcurrent::filtered()}, and
+    \l {QtConcurrent::filteredReduced}{QtConcurrent::filteredReduced()}
+    functions, you can include a more specific header:
+
+    \code
+    #include <QtConcurrentFilter>
+    \endcode
+
     \section1 Concurrent Filter
 
     QtConcurrent::filtered() takes an input sequence and a filter function.

src/concurrent/qtconcurrentmap.cpp

@@ -13,6 +13,38 @@
     See the \l {Qt Concurrent} module documentation for an overview of available
     functions, or see below for detailed information on each function.
 
+    \section1 Optimize includes
+
+    If you include the \c <QtConcurrent> header, the entire Qt Concurrent
+    module with the entire Qt Core module will be included, which may increase
+    compilation times and binary sizes. To use individual functions from the
+    QtConcurrent namespace, you can include more specific headers.
+
+    The table below lists the functions in the QtConcurrent namespace and
+    their corresponding headers:
+
+    \table
+    \header
+        \li Function
+        \li Header
+    \row
+        \li \l {QtConcurrent::run}{QtConcurrent::run()}
+        \li \c <QtConcurrentRun>
+    \row
+        \li \l {QtConcurrent::task}{QtConcurrent::task()}
+        \li \c <QtConcurrentTask>
+    \row
+        \li \l {QtConcurrent::filter}{QtConcurrent::filter()},
+            \l {QtConcurrent::filtered}{QtConcurrent::filtered()},
+            \l {QtConcurrent::filteredReduced}{QtConcurrent::filteredReduced()}
+        \li \c <QtConcurrentFilter>
+    \row
+        \li \l {QtConcurrent::map}{QtConcurrent::map()},
+            \l {QtConcurrent::mapped}{QtConcurrent::mapped()},
+            \l {QtConcurrent::mappedReduced}{QtConcurrent::mappedReduced()}
+        \li \c <QtConcurrentMap>
+    \endtable
+
     \inheaderfile QtConcurrent
     \ingroup thread
 */
@@ -130,6 +162,20 @@
     Note that the result types above are not QFuture objects, but real result
     types (in this case, QList<QImage> and QImage).
 
+    \section1 Optimize includes
+
+    If you include the \c <QtConcurrent> header, the entire Qt Concurrent
+    module with the entire Qt Core module will be included, which may increase
+    compilation times and binary sizes. To use the
+    \l {QtConcurrent::map}{QtConcurrent::map()},
+    \l {QtConcurrent::mapped}{QtConcurrent::mapped()}, and
+    \l {QtConcurrent::mappedReduced}{QtConcurrent::mappedReduced()} functions,
+    you can include a more specific header:
+
+    \code
+    #include <QtConcurrentMap>
+    \endcode
+
     \section1 Concurrent Map
 
     QtConcurrent::mapped() takes an input sequence and a map function. This map

src/concurrent/qtconcurrentrun.cpp

@@ -23,6 +23,18 @@
 
     This function is a part of the Qt Concurrent framework.
 
+    \section1 Optimize includes
+
+    If you include the \c <QtConcurrent> header, the entire Qt Concurrent
+    module with the entire Qt Core module will be included, which may increase
+    compilation times and binary sizes. To use the
+    \l {QtConcurrent::run}{QtConcurrent::run()} function, you can include a
+    more specific header:
+
+    \code
+    #include <QtConcurrentRun>
+    \endcode
+
     \section1 Concurrent Run (basic mode)
 
     The function passed to QtConcurrent::run() may report the result

src/concurrent/qtconcurrenttask.qdoc

@@ -18,6 +18,18 @@
 
     This function is a part of the \l {Qt Concurrent} framework.
 
+    \section1 Optimize includes
+
+    If you include the \c <QtConcurrent> header, the entire Qt Concurrent
+    module with the entire Qt Core module will be included, which may increase
+    compilation times and binary sizes. To use the
+    \l {QtConcurrent::task}{QtConcurrent::task()} function, you can include a
+    more specific header:
+
+    \code
+    #include <QtConcurrentTask>
+    \endcode
+
     \section1 Fluent interface
 
     The QtConcurrent::task returns an instance of an auxiliary class called