This PR was squashed before being merged into the 2.2.x-dev branch (closes #1570).

Discussion
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Update security.rst - security context sharing

I'd like to update the security.rst as I found the lack of information about the security context sharing between multiple firewalls. I know a lot of people are having problems not knowing this, so it would be nice to mention it. Please, check the correctness and approve it if you find it useful. Thanks.

Commits
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2b81227d Update security.rst - security context sharing