Improve spring-web filter documentation

[ciscoo]

Affects: 5.3.x, 6.0.x

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The Spring Web MVC filters section only mentions the provided filters by the framework.

Whereas the Spring WebFlux filters section explains that WebFilter can be used to implement filters for WebFlux.

I had expected to see in the Spring Web MVC docs to mention something similar for GenericFilterBean or OncePerRequestFilter that they can be used in a Servlet environment

[earosselot]

Hi, I would like to work on this issue. I've worked with filters before and this could be a good first PR for me, is that ok?

I'm thinking about present some explanation and one example for each of the abstract filters GenericFilterBean , OncePerRequestFilter (not sure if AbstractRequestLoggingFilter should be on docs too) with a customized beahvior.