Help/prop_test/ENVIRONMENT: clarify the scope of the changes

The "restored" bit is an implementation detail as it could also be
implemented by passing a crafted environment to `execve` or
`CreateProcess` arguments. Instead, state that the environment changes
only affects the test with the property set.

Help/prop_test/ENVIRONMENT.rst

@@ -5,5 +5,5 @@ Specify environment variables that should be defined for running a test.
 
 If set to a list of environment variables and values of the form
 ``MYVAR=value`` those environment variables will be defined while running
-the test.  The environment is restored to its previous state after the
-test is done.
+the test.  The environment changes from this property do not affect other
+tests.