Doc: Set Qt 5.9 as an LTS release

... And also do the same for Qt 5.6.

Change-Id: I9e7bde3043dff49e19bc455d130471dfc206ad0b
Reviewed-by: Kai Koehne <kai.koehne@qt.io>

doc/src/platforms/supported-platforms.qdocinc

@@ -40,7 +40,7 @@ Mobile Platforms: \l {Qt for Android}{Android},
 
 //! [all]
 \section1 Qt 5.9
-\b {Supported until one year after release}
+\b {Long-term support (LTS)}: Supported until three years after release
 
 //! [5.9] //! [latest]
 
@@ -272,7 +272,7 @@ Mobile Platforms: \l {Qt for Android}{Android},
     \section1 Qt 5.6
 
 //! [5.6]
-    \b {Supported until Mar. 16, 2019}
+    \b {Long-term support (LTS)}: Supported until Mar. 16, 2019
 
     \table 80%
     \header \li Platform \li Compiler \li Notes

doc/src/whatsnew/whatsnew56.qdoc

@@ -30,6 +30,8 @@
     \title What's New in Qt 5.6
     \brief Lists the new features in Qt 5.6.
 
+    Qt 5.6 is a long-term support (LTS) release.
+
     \section1 New Modules
 
     \list

doc/src/whatsnew/whatsnew59.qdoc

@@ -30,6 +30,8 @@
     \title What's New in Qt 5.9
     \brief Lists the new features in Qt 5.9.
 
+    Qt 5.9 is a long-term support (LTS) release.
+
     \section1 New Features in Qt 5.9
 
     \section2 Qt Core Module