docs: Updating 'custom' annotation docs
docs: Updating 'custom' annotation docs

to reflect that they are a mapping and not a list.

Signed-off-by: Johan Fylling <05c1a2f0e68460a354c87096e0c2d4ddf2525644@fylling.se>

open-policy-agent/opa,tsandall/opa,tsandall/opa,open-policy-agent/opa,open-policy-agent/opa,open-policy-agent/opa,tsandall/opa,tsandall/opa,tsandall/opa,tsandall/opa,open-policy-agent/opa,open-policy-agent/opa