CamelQuarkusTestSupport: Update doc about 'global' RouteBuilder behavior

Follows https://github.com/apache/camel-quarkus/issues/4560 and https://github.com/apache/camel-quarkus/issues/4569. Those issues unrevealed a behavior which is not obvious.

Let me explain.
In camel -> if you have a RouteBuilder (not created by testSupport class but global one), if you change anything on the route during a test. The following test will start **without** such a change.
In camel-quarkus -> if one test modifies a route from global routeBuilder, the following test will **inherit** this change

I think that a short sentence or mention of such behavior may help users to find a reason of unfamiliar behavior compared to camel.