Doc: add info about the -perf benchmark switch to QtTest's manual

Change-Id: I923dcf4f59db8281800950684fdd6dcc6715508e
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>

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

@@ -315,12 +315,15 @@
     \row \li CPU tick counter
          \li -tickcounter
          \li Windows, Mac OS X, Linux, many UNIX-like systems.
-    \row \li Valgrind Callgrind
-         \li -callgrind
-         \li Linux (if installed)
     \row \li Event Counter
          \li -eventcounter
          \li All platforms
+    \row \li Valgrind Callgrind
+         \li -callgrind
+         \li Linux (if installed)
+    \row \li Linux Perf
+         \li -perf
+         \li Linux
     \endtable
 
     In short, walltime is always available but requires many repetitions to
@@ -335,6 +338,16 @@
     that were received by the event loop before they are sent to their corresponding
     targets (this might include non-Qt events).
 
+    The Linux Performance Monitoring solution is available only on Linux and
+    provides many different counters, which can be selected by passing an
+    additional option \c {-perfcounter countername}, such as \c {-perfcounter
+    cache-misses}, \c {-perfcounter branch-misses}, or \c {-perfcounter
+    l1d-load-misses}. The default counter is \c {cpu-cycles}. The full list of
+    counters can be obtained by running any benchmark executable with the
+    option \c -perfcounterlist.
+
+    Note that using the performance counter may require enabling access to non-privileged applications.
+
     \note Depending on the device configuration, tick counters on the
      Windows CE platform may not be as fine-grained, compared to other platforms.
      Devices that do not support high-resolution timers default to