Update documentation of max_pyver test option (#5411)

Author: DanielNoord

doc/development_guide/testing.rst

@@ -76,7 +76,7 @@ The .rc file can also contain a section ``[testoptions]`` to pass options for th
 test runner. The following options are currently supported:
 
     "min_pyver": Minimal python version required to run the test
-    "max_pyver": Maximum python version required to run the test
+    "max_pyver": Python version from which the test won't be run. If the last supported version is 3.9 this setting should be set to 3.10.
     "min_pyver_end_position": Minimal python version required to check the end_line and end_column attributes of the message
     "requires": Packages required to be installed locally to run the test
     "except_implementations": List of python implementations on which the test should not run