Description of `securityMatcher` and multiple filter chains could use more details

[sjohnr]

It was noted in #14382 that the docs aren't clear on how different configurations that make use of securityMatcher with multiple filter chains affect the Spring Security defaults, such as generated login/logout pages, the default logout endpoint, etc. We should enhance the docs (for example here) to provide more clarity and possibly more examples.

[abimael-turing]

Hey @sjohnr, please assign this issue to me so I can take care of it. Thank you!

[abimael-turing]

@sjohnr, I have submitted a pull request. Please review it when you have a moment. Thank you.