Doc: Add Apple OS deployment targets to the qmake Manual

The following variables were not documented:

- QMAKE_IOS_DEPLOYMENT_TARGET
- QMAKE_TVOS_DEPLOYMENT_TARGET
- QMAKE_WATCHOS_DEPLOYMENT_TARGET

Change-Id: I5cfb6c0024d92e943aed882fd01bc2a4f2c7c042
Reviewed-by: Jake Petroules <jake.petroules@qt.io>

qmake/doc/src/qmake-manual.qdoc

@@ -1,6 +1,6 @@
 /****************************************************************************
 **
-** Copyright (C) 2016 The Qt Company Ltd.
+** Copyright (C) 2017 The Qt Company Ltd.
 ** Contact: https://www.qt.io/licensing/
 **
 ** This file is part of the documentation of the Qt Toolkit.
@@ -1809,6 +1809,14 @@
 
     \note Most of the time, the default \c{Info.plist} is good enough.
 
+    \section1 QMAKE_IOS_DEPLOYMENT_TARGET
+
+    \note This variable is used on the iOS platform only.
+
+    Specifies the hard minimum version of iOS that the application supports.
+
+    For more information, see \l {Expressing Supported iOS Versions}.
+
     \section1 QMAKE_LFLAGS
 
     Specifies a general set of flags that are passed to
@@ -2251,6 +2259,22 @@
     This is only utilized if the \l{VERSION} or \l{RC_ICONS} variable is set
     and the \l{RC_FILE} and \l{RES_FILE} variables are not set.
 
+    \section1 QMAKE_TVOS_DEPLOYMENT_TARGET
+
+    \note This variable is used on the tvOS platform only.
+
+    Specifies the hard minimum version of tvOS that the application supports.
+
+    For more information, see \l {Expressing Supported iOS Versions}.
+
+    \section1 QMAKE_WATCHOS_DEPLOYMENT_TARGET
+
+    \note This variable is used on the watchOS platform only.
+
+    Specifies the hard minimum version of watchOS that the application supports.
+
+    For more information, see \l {Expressing Supported iOS Versions}.
+
     \section1 QT
 
     Specifies the \l{All Modules}{Qt modules} that are used by your project. For