Doc: Add best-practices-info about creating benchmarks

From https://wiki.qt.io/Writing_Unit_Tests

Change-Id: Idc0bafb32690f443e1f49cd4f8efb653d3aa46a4
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>

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

@@ -317,10 +317,35 @@
     \section1 Creating a Benchmark
 
     To create a benchmark, follow the instructions for creating a test and then add a
-    QBENCHMARK macro to the test function that you want to benchmark.
+    \l QBENCHMARK macro or \l QTest::setBenchmarkResult() to the test function that
+    you want to benchmark. In the following code snippet, the macro is used:
 
     \snippet code/doc_src_qtestlib.cpp 12
 
+    A test function that measures performance should contain either a single
+    \c QBENCHMARK macro or a single call to \c setBenchmarkResult(). Multiple
+    occurrences make no sense, because only one performance result can be
+    reported per test function, or per data tag in a data-driven setup.
+
+    Avoid changing the test code that forms (or influences) the body of a
+    \c QBENCHMARK macro, or the test code that computes the value passed to
+    \c setBenchmarkResult(). Differences in successive performance results
+    should ideally be caused only by changes to the product you are testing.
+    Changes to the test code can potentially result in misleading report of
+    a change in performance. If you do need to change the test code, make
+    that clear in the commit message.
+
+    In a performance test function, the \c QBENCHMARK or \c setBenchmarkResult()
+    should be followed by a verification step using \l QCOMPARE(), \l QVERIFY(),
+    and so on. You can then flag a performance result as \e invalid if another
+    code path than the intended one was measured. A performance analysis tool
+    can use this information to filter out invalid results.
+    For example, an unexpected error condition will typically cause the program
+    to bail out prematurely from the normal program execution, and thus falsely
+    show a dramatic performance increase.
+
+    \section2 Selecting the Measurement Back-end
+
     The code inside the QBENCHMARK macro will be measured, and possibly also repeated
     several times in order to get an accurate measurement. This depends on the selected
     measurement back-end. Several back-ends are available. They can be selected on the