Add note on selecting the device which is used to run tests on Android

Pick-to: 6.2 5.15
Change-Id: I9bcff18ca11fbbfdff968e29190cae488de56263
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>

doc/global/externalsites/external-resources.qdoc

@@ -40,6 +40,10 @@
     \externalpage https://developer.android.com/studio/publish/versioning#appversioning
     \title Android: App Versioning
 */
+/*!
+    \externalpage https://developer.android.com/studio/command-line/adb#devicestatus
+    \title Android: Query for devices
+*/
 /*!
     \externalpage http://www.freedesktop.org/
     \title freedesktop.org

src/testlib/doc/src/qttestlib-manual.qdoc

@@ -153,6 +153,12 @@
     All labeled targets will be run when \c {test} target is called on the
     command line.
 
+    \note On Android, if you have one connected device or emulator, tests will
+    run on that device. If you have more than one device connected, set the
+    environment variable \c {ANDROID_DEVICE_SERIAL} to the
+    \l {Android: Query for devices}{ADB serial number} of the device that
+    you want to run tests on.
+
     There are several other advantages with CMake. For example, the result of
     a test run can be published on a web server using CDash with virtually no
     effort.