[doc] Improve documentation about setting performance references in tests

reframe/core/pipeline.py

@@ -609,6 +609,25 @@ def pipeline_hooks(cls):
     #:        }
     #:    }
     #:
+    #: To better understand how to set the performance reference tuple, here
+    #: are some examples with both positive and negative reference values:
+    #:
+    #:   ============================== ============  ==========  ===========
+    #:   **Performance Tuple**          **Expected**  **Lowest**  **Highest**
+    #:   ``(100, -0.01, 0.02, 'MB/s')`` 100 MB/s      99 MB/s     102 MB/s
+    #:   ``(100, -0.01, None, 'MB/s')`` 100 MB/s      99 MB/s     inf MB/s
+    #:   ``(100, None, 0.02, 'MB/s')``  100 MB/s      -inf MB/s   102 MB/s
+    #:   ``(-100, -0.01, 0.02, 'C')``     -100 C        -101 C      -98 C
+    #:   ``(-100, -0.01, None, 'C')``     -100 C        -101 C      inf C
+    #:   ``(-100, None, 0.02, 'C')``      -100 C        -inf C      -98 C
+    #:   ============================== ============  ==========  ===========
+    #:
+    #: During the performance stage of the pipeline, the reference tuple
+    #: elements, except the unit, are passed to the
+    #: :func:`~reframe.utility.sanity.assert_reference` function along with the
+    #: obtained performance value in order to actually assess whether the test
+    #: passes the performance check or not.
+    #:
     #: :type: A scoped dictionary with system names as scopes or :class:`None`
     #: :default: ``{}``
     #: