Document removal of macOS 32-bit support

32-bit support has been deprecated for some time,
and several breaking changes have gone into the 5.10
branch.

Make the current state official: Qt 5.10 and higher
will not support 32-bit mode. Qt 5.9 LTS will continue
to do so.

Change-Id: Iaea4f3499054093984ad70cb60b837fb506ca16e
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>
Reviewed-by: Jake Petroules <jake.petroules@qt.io>

doc/src/platforms/osx.qdoc

@@ -63,19 +63,20 @@
 
     See \l{Supported Platforms} for the list of \macos versions supported by Qt.
 
-    Qt can be built for either x86 or x86_64. 64-bit is used by default.
-    To select a 32-bit build, use the \c QMAKE_APPLE_DEVICE_ARCHS \c qmake variable.
+    Qt can be built for either x86_64 or x86_64h (Haswell). x86_64 is used by default.
+    To select a x86_64h build, use the \c QMAKE_APPLE_DEVICE_ARCHS \c qmake variable.
     This is selectable at configure time:
     \code
-    ./configure -platform macx-clang QMAKE_APPLE_DEVICE_ARCHS=i386
+    ./configure -platform macx-clang QMAKE_APPLE_DEVICE_ARCHS=x86_64h
     \endcode
 
     \c QMAKE_APPLE_DEVICE_ARCHS can also be specified as a space-delimited list
     in order to build for multiple architectures simultaneously:
     \code
-    ./configure -platform macx-clang QMAKE_APPLE_DEVICE_ARCHS="i386 x86_64 x86_64h"
+    ./configure -platform macx-clang QMAKE_APPLE_DEVICE_ARCHS="x86_64 x86_64h"
     \endcode
 
+    \note Qt 5.10+ does not support 32-bit builds.
     \note Qt 5 does not support OS X on PowerPC.
     \note Static builds are not tested.