Doc: Describe the Enable high DPI scaling option

Task-number: QTCREATORBUG-20232
Change-Id: I3a0ba2fcfd08fe6f3208754f369241553e0d0d83
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@qt.io>
Reviewed-by: Mitch Curtis <mitch.curtis@qt.io>

doc/src/howto/creator-ui.qdoc

@@ -601,6 +601,11 @@
 
     \section1 Platform Notes
 
+    This section describes the cases where the behavior of \QC depends on the
+    operating system it runs on.
+
+    \section2 Locations of Functions
+
     \QC uses standard names and locations for standard features, such as
     \e options or \e preferences. In this manual, the names and locations on
     Windows and Linux are usually used to keep the instructions short. Here are
@@ -623,4 +628,29 @@
                 \uicontrol Keyboard
     \endtable
 
+    \section2 High DPI Scaling
+
+    The operating systems supported by \QC implement high dots-per-inch (DPI)
+    scaling at varying levels. Therefore, \QC handles high DPI scaling
+    differently on different operating system:
+
+    \list
+
+        \li On \macos, high DPI scaling is forced, which means that \QC allows
+            Qt to use the system scaling factor as the \QC scaling factor.
+
+        \li On Windows, if no \l{High DPI Support in Qt}
+            {scaling environment variables} are set, \QC instructs Qt to detect
+            the scaling factor and use it for \QC.
+
+        \li On Linux, \QC leaves it to the user to enable high DPI scaling,
+            because the process varies so much on different distributions
+            and windowing systems that it cannot be reliably done automatically.
+
+    \endlist
+
+    To override the default approach and always enable high-DPI scaling, select
+    \uicontrol Tools > \uicontrol Options > \uicontrol Environment >
+    \uicontrol {Enable high DPI scaling}. The changes will take effect after you
+    restart \QC.
 */