Doc: Fixed information about the default modules when using qmake.

-Qt GUI and Qt Core are included by default
-Added entry about disabling "gui" in the .pro file
-Information placed in snippet files

Change-Id: I1c70dcac9ca2d443e9f658e2a27727d24a25cae1
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>

src/corelib/doc/src/qtcore-index.qdoc

@@ -37,6 +37,8 @@
 
     \snippet code/doc_src_qtcore.cpp 0
 
+    If you use \l qmake to build your projects, Qt Core is included by default.
+
     \section1 Core Functionalities
 
     Qt adds these features to C++:

src/corelib/doc/src/qtcore.qdoc

@@ -37,4 +37,8 @@
     definitions of the module's classes, use the following directive:
 
     \snippet code/doc_src_qtcore.cpp 0
+
+    If you use \l qmake to build your projects, \l{Qt Core} is included by
+    default.
+
 */

src/gui/doc/snippets/code/doc_src_qtgui.pro

@@ -1,3 +1,7 @@
 #! [0]
 #include <QtGui>
 #! [0]
+
+#! [1]
+QT -= gui
+#! [1]

src/gui/doc/src/qtgui.qdoc

@@ -46,7 +46,10 @@
 
     \snippet code/doc_src_qtgui.pro 0
 
-    See the \l {Qt GUI} page for more details.
+    If you use \l qmake to build your projects, \l{Qt GUI} is included by
+    default. To disable Qt GUI, add the following line to your \c .pro file:
+
+    \snippet code/doc_src_qtgui.pro 1
 
 */
 
@@ -72,7 +75,10 @@
 
     \snippet code/doc_src_qtgui.pro 0
 
+    If you use \l qmake to build your projects, Qt GUI is included by
+    default. To disable Qt GUI, add the following line to your \c .pro file:
 
+    \snippet code/doc_src_qtgui.pro 1
 
     \section1 Application Windows