Autocert: Add more docs for restrictCertificatesToNamespace

[sourishkrout]

What would you like to be added

Now that autocert can restrict certificates SANs to only be issued within its respective namespace documentation is required to explain the feature and its usage. Let's add this to the docs.

Why this is needed

While this feature is beneficial to enforce namespace-level security, it might have impacts on the user experience which is why we want to highlight it better in the docs.

Potentially include docs in runbooks.md and/or FAQs