Doc: Fix documentation for devicePixelRatio() functions

Fix Incorrect use of \sa, missing markup for comment block and typos.

Change-Id: I1b7e477b7429ace30ca3ab25c4814c8cba659a65
Reviewed-by: Venugopal Shivashankar <venugopal.shivashankar@digia.com>
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>

src/gui/kernel/qguiapplication.cpp

@@ -709,10 +709,9 @@ QList<QScreen *> QGuiApplication::screens()
     device-independent pixels.
 
     Use this function only when you don't know which window you are targeting.
-    If you do know the target window use QWindow::devicePixelRatio() instead.
+    If you do know the target window, use QWindow::devicePixelRatio() instead.
 
-    \sa QWindow::devicePixelRatio();
-    \sa QGuiApplicaiton::devicePixelRatio();
+    \sa QWindow::devicePixelRatio()
 */
 qreal QGuiApplication::devicePixelRatio() const
 {

src/gui/kernel/qscreen.cpp

@@ -214,13 +214,12 @@ qreal QScreen::logicalDotsPerInch() const
     return (dpi.first + dpi.second) * qreal(0.5);
 }
 
-/*
+/*!
     Returns the ratio between physical pixels and device-independent pixels for the screen.
 
-    Common values are 1.0 on normal displays and 2.0 on Apple retina displays.
+    Common values are 1.0 on normal displays and 2.0 on Apple "retina" displays.
 
-    \sa QWindow::devicePixelRatio();
-    \sa QGuiApplicaiton::devicePixelRatio();
+    \sa QWindow::devicePixelRatio(), QGuiApplication::devicePixelRatio()
 */
 qreal QScreen::devicePixelRatio() const
 {

src/gui/kernel/qwindow.cpp

@@ -765,8 +765,7 @@ Qt::ScreenOrientation QWindow::contentOrientation() const
 
     Common values are 1.0 on normal displays and 2.0 on Apple "retina" displays.
 
-    \sa QWindow::devicePixelRatio();
-    \sa QGuiApplicaiton::devicePixelRatio();
+    \sa QScreen::devicePixelRatio(), QGuiApplication::devicePixelRatio()
 */
 qreal QWindow::devicePixelRatio() const
 {