add hint for class attributes

solves [8252](pytest-dev#8252)

Author: christian-steinmeyer

doc/en/getting-started.rst

@@ -210,6 +210,8 @@ This is outlined below:
     FAILED test_class_demo.py::TestClassDemoInstance::test_two - assert 0
     2 failed in 0.12s
 
+Note, that attributes added at class level (e.g. outside of the `setup_method(self)`), will be shared between tests.
+
 Request a unique temporary directory for functional tests
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