chg: doc: Add non-IN RR classes to list of unsupported configurations

This adds a note to the security documentation that configuring
resources with non-Internet DNS classes (CHAOS, HESIOD, ...) is not
a supported configuration and could potentially cause issues.

Closes #5805

Merge branch '5805-add-clarification-on-non-IN-classes' into 'main'

See merge request isc-projects/bind9!11667